| C++ Mathematical Expression Toolkit Library Documentation 
  |  
  |   Section 00 - Introduction 
  |   Section 01 - Capabilities 
  |   Section 02 - Example Expressions 
  |   Section 03 - Copyright Notice 
  |   Section 04 - Downloads & Updates 
  |   Section 05 - Installation 
  |   Section 06 - Compilation 
  |   Section 07 - Compiler Compatibility 
  |   Section 08 - Built-In Operations & Functions 
  |   Section 09 - Fundamental Types 
  |   Section 10 - Components 
  |   Section 11 - Compilation Options 
  |   Section 12 - Expression Structures 
  |   Section 13 - Variable, Vector & String Definition 
  |   Section 14 - Vector Processing 
  |   Section 15 - User Defined Functions 
  |   Section 16 - Expression Dependents 
  |   Section 17 - Hierarchies Of Symbol Tables 
  |   Section 18 - Unknown Unknowns 
  |   Section 19 - Enabling & Disabling Features 
  |   Section 20 - Expression Return Values 
  |   Section 21 - Compilation Errors 
  |   Section 22 - Runtime Library Packages 
  |   Section 23 - Helpers & Utils 
  |   Section 24 - Runtime Checks 
  |   Section 25 - Benchmarking 
  |   Section 26 - Exprtk Notes 
  |   Section 27 - Simple Exprtk Example 
  |   Section 28 - Build Options 
  |   Section 29 - Files 
  |   Section 30 - Language Structure 
  |  
  |  
  | [SECTION 00 - INTRODUCTION] 
  | The C++ Mathematical Expression  Toolkit Library (ExprTk) is  a simple 
  | to  use,   easy  to   integrate  and   extremely  efficient   run-time 
  | mathematical  expression parsing  and evaluation  engine. The  parsing 
  | engine  supports numerous  forms  of  functional and  logic processing 
  | semantics and is easily extensible. 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 01 - CAPABILITIES] 
  | The  ExprTk expression  evaluator supports  the following  fundamental 
  | arithmetic operations, functions and processes: 
  |  
  |  (00) Types:           Scalar, Vector, String 
  |  
  |  (01) Basic operators: +, -, *, /, %, ^ 
  |  
  |  (02) Assignment:      :=, +=, -=, *=, /=, %= 
  |  
  |  (03) Equalities & 
  |       Inequalities:    =, ==, <>, !=, <, <=, >, >= 
  |  
  |  (04) Logic operators: and, mand, mor, nand, nor, not, or, shl, shr, 
  |                        xnor, xor, true, false 
  |  
  |  (05) Functions:       abs, avg, ceil, clamp, equal, erf, erfc,  exp, 
  |                        expm1, floor, frac,  log, log10, log1p,  log2, 
  |                        logn, max,  min, mul,  ncdf,  not_equal, root, 
  |                        round, roundn, sgn, sqrt, sum, swap, trunc 
  |  
  |  (06) Trigonometry:    acos, acosh, asin, asinh, atan, atanh,  atan2, 
  |                        cos,  cosh, cot,  csc, sec,  sin, sinc,  sinh, 
  |                        tan, tanh, hypot, rad2deg, deg2grad,  deg2rad, 
  |                        grad2deg 
  |  
  |  (07) Control 
  |       structures:      if-then-else, ternary conditional, switch-case, 
  |                        return-statement 
  |  
  |  (08) Loop statements: while, for, repeat-until, break, continue 
  |  
  |  (09) String 
  |       processing:      in, like, ilike, concatenation 
  |  
  |  (10) Optimisations:   constant-folding, simple strength reduction and 
  |                        dead code elimination 
  |  
  |  (11) Runtime checks:  vector bounds, string bounds, loop iteration, 
  |                        execution-time bounds and compilation process 
  |                        checkpointing, assert statements 
  |  
  |  (12) Calculus:        numerical integration and differentiation 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 02 - EXAMPLE EXPRESSIONS] 
  | The following is  a short listing  of infix format  based mathematical 
  | expressions that can be parsed and evaluated using the ExprTk library. 
  |  
  |   (01) sqrt(1 - (3 / x^2)) 
  |   (02) clamp(-1, sin(2 * pi * x) + cos(y / 2 * pi), +1) 
  |   (03) sin(2.34e-3 * x) 
  |   (04) if(((x[2] + 2) == 3) and ((y + 5) <= 9),1 + w, 2 / z) 
  |   (05) inrange(-2,m,+2) == if(({-2 <= m} and [m <= +2]),1,0) 
  |   (06) ({1/1}*[1/2]+(1/3))-{1/4}^[1/5]+(1/6)-({1/7}+[1/8]*(1/9)) 
  |   (07) a * exp(2.2 / 3.3 * t) + c 
  |   (08) z := x + sin(2.567 * pi / y) 
  |   (09) u := 2.123 * {pi * z} / (w := x + cos(y / pi)) 
  |   (10) 2x + 3y + 4z + 5w == 2 * x + 3 * y + 4 * z + 5 * w 
  |   (11) 3(x + y) / 2.9 + 1.234e+12 == 3 * (x + y) / 2.9 + 1.234e+12 
  |   (12) (x + y)3.3 + 1 / 4.5 == [x + y] * 3.3 + 1 / 4.5 
  |   (13) (x + y[i])z + 1.1 / 2.7 == (x + y[i]) * z + 1.1 / 2.7 
  |   (14) (sin(x / pi) cos(2y) + 1) == (sin(x / pi) * cos(2 * y) + 1) 
  |   (15) 75x^17 + 25.1x^5 - 35x^4 - 15.2x^3 + 40x^2 - 15.3x + 1 
  |   (16) (avg(x,y) <= x + y ? x - y : x * y) + 2.345 * pi / x 
  |   (17) while (x <= 100) { x -= 1; } 
  |   (18) x <= 'abc123' and (y in 'AString') or ('1x2y3z' != z) 
  |   (19) ((x + 'abc') like '*123*') or ('a123b' ilike y) 
  |   (20) sgn(+1.2^3.4z / -5.6y) <= {-7.8^9 / -10.11x } 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 03 - COPYRIGHT NOTICE] 
  | Free  use  of  the  C++  Mathematical  Expression  Toolkit  Library is 
  | permitted under the guidelines and in accordance with the most current 
  | version of the MIT License. 
  |  
  |    (1) https://www.opensource.org/licenses/MIT 
  |    (2) SPDX-License-Identifier: MIT 
  |    (3) SPDX-FileCopyrightText : Copyright (C) 1999-2024 Arash Partow 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 04 - DOWNLOADS & UPDATES] 
  | The most  recent version  of the C++ Mathematical  Expression  Toolkit 
  | Library including all updates and tests can be found at the  following 
  | locations: 
  |  
  |   (1) Download: https://www.partow.net/programming/exprtk/index.html 
  |   (2) Mirror Repository: https://github.com/ArashPartow/exprtk 
  |                          https://github.com/ArashPartow/exprtk-extras 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 05 - INSTALLATION] 
  | The header  file exprtk.hpp  should be  placed in a project or  system 
  | include path (e.g: /usr/include/). 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 06 - COMPILATION] 
  | The  ExprTk  package  contains  the ExprTk  header,  a  set  of simple 
  | examples and a benchmark and unit test  suite. The following is a list 
  | of commands to build the various components: 
  |  
  |   (a) For a complete build: make clean all 
  |   (b) For a PGO build: make clean pgo 
  |   (c) To strip executables: make strip_bin 
  |   (d) Execute valgrind check: make valgrind_check 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 07 - COMPILER COMPATIBILITY] 
  | ExprTk has been built error and warning free using the following set 
  | of C++ compilers: 
  |  
  |   (*) GNU Compiler Collection (3.5+) 
  |   (*) Clang/LLVM (1.1+) 
  |   (*) Microsoft Visual Studio C++ Compiler (7.1+) 
  |   (*) Intel C++ Compiler (8.x+) 
  |   (*) AMD Optimizing C++ Compiler (1.2+) 
  |   (*) Nvidia C++ Compiler (19.x+) 
  |   (*) PGI C++ (10.x+) 
  |   (*) Circle C++ (circa: b81c37d2bb227c) 
  |   (*) IBM XL C/C++ (9.x+) 
  |   (*) C++ Builder (XE4+) 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 08 - BUILT-IN OPERATIONS & FUNCTIONS] 
  |  
  | (0) Arithmetic & Assignment Operators 
  | +----------+---------------------------------------------------------+ 
  | | OPERATOR | DEFINITION                                              | 
  | +----------+---------------------------------------------------------+ 
  | |  +       | Addition between x and y.  (eg: x + y)                  | 
  | +----------+---------------------------------------------------------+ 
  | |  -       | Subtraction between x and y.  (eg: x - y)               | 
  | +----------+---------------------------------------------------------+ 
  | |  *       | Multiplication between x and y.  (eg: x * y)            | 
  | +----------+---------------------------------------------------------+ 
  | |  /       | Division between x and y.  (eg: x / y)                  | 
  | +----------+---------------------------------------------------------+ 
  | |  %       | Modulus of x with respect to y.  (eg: x % y)            | 
  | +----------+---------------------------------------------------------+ 
  | |  ^       | x to the power of y.  (eg: x ^ y)                       | 
  | +----------+---------------------------------------------------------+ 
  | |  :=      | Assign the value of x to y. Where y is either a variable| 
  | |          | or vector type.  (eg: y := x)                           | 
  | +----------+---------------------------------------------------------+ 
  | |  +=      | Increment x by the value of the expression on the right | 
  | |          | hand side. Where x is either a variable or vector type. | 
  | |          | (eg: x += abs(y - z))                                   | 
  | +----------+---------------------------------------------------------+ 
  | |  -=      | Decrement x by the value of the expression on the right | 
  | |          | hand side. Where x is either a variable or vector type. | 
  | |          | (eg: x[i] -= abs(y + z))                                | 
  | +----------+---------------------------------------------------------+ 
  | |  *=      | Assign the multiplication of x by the value of the      | 
  | |          | expression on the righthand side to x. Where x is either| 
  | |          | a variable or vector type.                              | 
  | |          | (eg: x *= abs(y / z))                                   | 
  | +----------+---------------------------------------------------------+ 
  | |  /=      | Assign the division of x by the value of the expression | 
  | |          | on the right-hand side to x. Where x is either a        | 
  | |          | variable or vector type.  (eg: x[i + j] /= abs(y * z))  | 
  | +----------+---------------------------------------------------------+ 
  | |  %=      | Assign x modulo the value of the expression on the right| 
  | |          | hand side to x. Where x is either a variable or vector  | 
  | |          | type.  (eg: x[2] %= y ^ 2)                              | 
  | +----------+---------------------------------------------------------+ 
  |  
  | (1) Equalities & Inequalities 
  | +----------+---------------------------------------------------------+ 
  | | OPERATOR | DEFINITION                                              | 
  | +----------+---------------------------------------------------------+ 
  | | == or =  | True only if x is strictly equal to y. (eg: x == y)     | 
  | +----------+---------------------------------------------------------+ 
  | | <> or != | True only if x does not equal y. (eg: x <> y or x != y) | 
  | +----------+---------------------------------------------------------+ 
  | |  <       | True only if x is less than y. (eg: x < y)              | 
  | +----------+---------------------------------------------------------+ 
  | |  <=      | True only if x is less than or equal to y. (eg: x <= y) | 
  | +----------+---------------------------------------------------------+ 
  | |  >       | True only if x is greater than y. (eg: x > y)           | 
  | +----------+---------------------------------------------------------+ 
  | |  >=      | True only if x greater than or equal to y. (eg: x >= y) | 
  | +----------+---------------------------------------------------------+ 
  |  
  | (2) Boolean Operations 
  | +----------+---------------------------------------------------------+ 
  | | OPERATOR | DEFINITION                                              | 
  | +----------+---------------------------------------------------------+ 
  | | true     | True state or any value other than zero (typically 1).  | 
  | +----------+---------------------------------------------------------+ 
  | | false    | False state, value of exactly zero.                     | 
  | +----------+---------------------------------------------------------+ 
  | | and      | Logical AND, True only if x and y are both true.        | 
  | |          | (eg: x and y)                                           | 
  | +----------+---------------------------------------------------------+ 
  | | mand     | Multi-input logical AND, True only if all inputs are    | 
  | |          | true. Left to right short-circuiting of expressions.    | 
  | |          | (eg: mand(x > y, z < w, u or v, w and x))               | 
  | +----------+---------------------------------------------------------+ 
  | | mor      | Multi-input logical OR, True if at least one of the     | 
  | |          | inputs are true. Left to right short-circuiting of      | 
  | |          | expressions.  (eg: mor(x > y, z < w, u or v, w and x))  | 
  | +----------+---------------------------------------------------------+ 
  | | nand     | Logical NAND, True only if either x or y is false.      | 
  | |          | (eg: x nand y)                                          | 
  | +----------+---------------------------------------------------------+ 
  | | nor      | Logical NOR, True only if the result of x or y is false | 
  | |          | (eg: x nor y)                                           | 
  | +----------+---------------------------------------------------------+ 
  | | not      | Logical NOT, Negate the logical sense of the input.     | 
  | |          | (eg: not(x and y) == x nand y)                          | 
  | +----------+---------------------------------------------------------+ 
  | | or       | Logical OR, True if either x or y is true. (eg: x or y) | 
  | +----------+---------------------------------------------------------+ 
  | | xor      | Logical XOR, True only if the logical states of x and y | 
  | |          | differ.  (eg: x xor y)                                  | 
  | +----------+---------------------------------------------------------+ 
  | | xnor     | Logical XNOR, True iff the biconditional of x and y is  | 
  | |          | satisfied.  (eg: x xnor y)                              | 
  | +----------+---------------------------------------------------------+ 
  | | &        | Similar to AND but with left to right expression short  | 
  | |          | circuiting optimisation.  (eg: (x & y) == (y and x))    | 
  | +----------+---------------------------------------------------------+ 
  | | |        | Similar to OR but with left to right expression short   | 
  | |          | circuiting optimisation.  (eg: (x | y) == (y or x))     | 
  | +----------+---------------------------------------------------------+ 
  |  
  | (3) General Purpose Functions 
  | +----------+---------------------------------------------------------+ 
  | | FUNCTION | DEFINITION                                              | 
  | +----------+---------------------------------------------------------+ 
  | | abs      | Absolute value of x.  (eg: abs(x))                      | 
  | +----------+---------------------------------------------------------+ 
  | | avg      | Average of all the inputs.                              | 
  | |          | (eg: avg(x,y,z,w,u,v) == (x + y + z + w + u + v) / 6)   | 
  | +----------+---------------------------------------------------------+ 
  | | ceil     | Smallest integer that is greater than or equal to x.    | 
  | +----------+---------------------------------------------------------+ 
  | | clamp    | Clamp x in range between r0 and r1, where r0 < r1.      | 
  | |          | (eg: clamp(r0,x,r1))                                    | 
  | +----------+---------------------------------------------------------+ 
  | | equal    | Equality test between x and y using normalised epsilon  | 
  | +----------+---------------------------------------------------------+ 
  | | erf      | Error function of x.  (eg: erf(x))                      | 
  | +----------+---------------------------------------------------------+ 
  | | erfc     | Complimentary error function of x.  (eg: erfc(x))       | 
  | +----------+---------------------------------------------------------+ 
  | | exp      | e to the power of x.  (eg: exp(x))                      | 
  | +----------+---------------------------------------------------------+ 
  | | expm1    | e to the power of x minus 1, where x is very small.     | 
  | |          | (eg: expm1(x))                                          | 
  | +----------+---------------------------------------------------------+ 
  | | floor    | Largest integer that is less than or equal to x.        | 
  | |          | (eg: floor(x))                                          | 
  | +----------+---------------------------------------------------------+ 
  | | frac     | Fractional portion of x.  (eg: frac(x))                 | 
  | +----------+---------------------------------------------------------+ 
  | | hypot    | Hypotenuse of x and y (eg: hypot(x,y) = sqrt(x*x + y*y))| 
  | +----------+---------------------------------------------------------+ 
  | | iclamp   | Inverse-clamp x outside of the range r0 and r1. Where   | 
  | |          | r0 < r1. If x is within the range it will snap to the   | 
  | |          | closest bound. (eg: iclamp(r0,x,r1)                     | 
  | +----------+---------------------------------------------------------+ 
  | | inrange  | In-range returns 'true' when x is within the range r0   | 
  | |          | and r1. Where r0 < r1.  (eg: inrange(r0,x,r1)           | 
  | +----------+---------------------------------------------------------+ 
  | | log      | Natural logarithm of x.  (eg: log(x))                   | 
  | +----------+---------------------------------------------------------+ 
  | | log10    | Base 10 logarithm of x.  (eg: log10(x))                 | 
  | +----------+---------------------------------------------------------+ 
  | | log1p    | Natural logarithm of 1 + x, where x is very small.      | 
  | |          | (eg: log1p(x))                                          | 
  | +----------+---------------------------------------------------------+ 
  | | log2     | Base 2 logarithm of x.  (eg: log2(x))                   | 
  | +----------+---------------------------------------------------------+ 
  | | logn     | Base N logarithm of x. where n is a positive integer.   | 
  | |          | (eg: logn(x,8))                                         | 
  | +----------+---------------------------------------------------------+ 
  | | max      | Largest value of all the inputs. (eg: max(x,y,z,w,u,v)) | 
  | +----------+---------------------------------------------------------+ 
  | | min      | Smallest value of all the inputs. (eg: min(x,y,z,w,u))  | 
  | +----------+---------------------------------------------------------+ 
  | | mul      | Product of all the inputs.                              | 
  | |          | (eg: mul(x,y,z,w,u,v,t) == (x * y * z * w * u * v * t)) | 
  | +----------+---------------------------------------------------------+ 
  | | ncdf     | Normal cumulative distribution function.  (eg: ncdf(x)) | 
  | +----------+---------------------------------------------------------+ 
  | | not_equal| Not-equal test between x and y using normalised epsilon | 
  | +----------+---------------------------------------------------------+ 
  | | pow      | x to the power of y.  (eg: pow(x,y) == x ^ y)           | 
  | +----------+---------------------------------------------------------+ 
  | | root     | Nth-Root of x. where n is a positive integer.           | 
  | |          | (eg: root(x,3) == x^(1/3))                              | 
  | +----------+---------------------------------------------------------+ 
  | | round    | Round x to the nearest integer.  (eg: round(x))         | 
  | +----------+---------------------------------------------------------+ 
  | | roundn   | Round x to n decimal places  (eg: roundn(x,3))          | 
  | |          | where n > 0 and is an integer.                          | 
  | |          | (eg: roundn(1.2345678,4) == 1.2346)                     | 
  | +----------+---------------------------------------------------------+ 
  | | sgn      | Sign of x, -1 where x < 0, +1 where x > 0, else zero.   | 
  | |          | (eg: sgn(x))                                            | 
  | +----------+---------------------------------------------------------+ 
  | | sqrt     | Square root of x, where x >= 0.  (eg: sqrt(x))          | 
  | +----------+---------------------------------------------------------+ 
  | | sum      | Sum of all the inputs.                                  | 
  | |          | (eg: sum(x,y,z,w,u,v,t) == (x + y + z + w + u + v + t)) | 
  | +----------+---------------------------------------------------------+ 
  | | swap     | Swap the values of the variables x and y and return the | 
  | | <=>      | current value of y.  (eg: swap(x,y) or x <=> y)         | 
  | +----------+---------------------------------------------------------+ 
  | | trunc    | Integer portion of x.  (eg: trunc(x))                   | 
  | +----------+---------------------------------------------------------+ 
  |  
  | (4) Trigonometry Functions 
  | +----------+---------------------------------------------------------+ 
  | | FUNCTION | DEFINITION                                              | 
  | +----------+---------------------------------------------------------+ 
  | | acos     | Arc cosine of x expressed in radians. Interval [-1,+1]  | 
  | |          | (eg: acos(x))                                           | 
  | +----------+---------------------------------------------------------+ 
  | | acosh    | Inverse hyperbolic cosine of x expressed in radians.    | 
  | |          | (eg: acosh(x))                                          | 
  | +----------+---------------------------------------------------------+ 
  | | asin     | Arc sine of x expressed in radians. Interval [-1,+1]    | 
  | |          | (eg: asin(x))                                           | 
  | +----------+---------------------------------------------------------+ 
  | | asinh    | Inverse hyperbolic sine of x expressed in radians.      | 
  | |          | (eg: asinh(x))                                          | 
  | +----------+---------------------------------------------------------+ 
  | | atan     | Arc tangent of x expressed in radians. Interval [-1,+1] | 
  | |          | (eg: atan(x))                                           | 
  | +----------+---------------------------------------------------------+ 
  | | atan2    | Arc tangent of (x / y) expressed in radians. [-pi,+pi]  | 
  | |          | eg: atan2(x,y)                                          | 
  | +----------+---------------------------------------------------------+ 
  | | atanh    | Inverse hyperbolic tangent of x expressed in radians.   | 
  | |          | (eg: atanh(x))                                          | 
  | +----------+---------------------------------------------------------+ 
  | | cos      | Cosine of x.  (eg: cos(x))                              | 
  | +----------+---------------------------------------------------------+ 
  | | cosh     | Hyperbolic cosine of x.  (eg: cosh(x))                  | 
  | +----------+---------------------------------------------------------+ 
  | | cot      | Cotangent of x.  (eg: cot(x))                           | 
  | +----------+---------------------------------------------------------+ 
  | | csc      | Cosecant of x.  (eg: csc(x))                            | 
  | +----------+---------------------------------------------------------+ 
  | | sec      | Secant of x.  (eg: sec(x))                              | 
  | +----------+---------------------------------------------------------+ 
  | | sin      | Sine of x.  (eg: sin(x))                                | 
  | +----------+---------------------------------------------------------+ 
  | | sinc     | Sine cardinal of x.  (eg: sinc(x))                      | 
  | +----------+---------------------------------------------------------+ 
  | | sinh     | Hyperbolic sine of x.  (eg: sinh(x))                    | 
  | +----------+---------------------------------------------------------+ 
  | | tan      | Tangent of x.  (eg: tan(x))                             | 
  | +----------+---------------------------------------------------------+ 
  | | tanh     | Hyperbolic tangent of x.  (eg: tanh(x))                 | 
  | +----------+---------------------------------------------------------+ 
  | | deg2rad  | Convert x from degrees to radians.  (eg: deg2rad(x))    | 
  | +----------+---------------------------------------------------------+ 
  | | deg2grad | Convert x from degrees to gradians.  (eg: deg2grad(x))  | 
  | +----------+---------------------------------------------------------+ 
  | | rad2deg  | Convert x from radians to degrees.  (eg: rad2deg(x))    | 
  | +----------+---------------------------------------------------------+ 
  | | grad2deg | Convert x from gradians to degrees.  (eg: grad2deg(x))  | 
  | +----------+---------------------------------------------------------+ 
  |  
  | (5) String Processing 
  | +----------+---------------------------------------------------------+ 
  | | FUNCTION | DEFINITION                                              | 
  | +----------+---------------------------------------------------------+ 
  | |  = , ==  | All common equality/inequality operators are applicable | 
  | |  !=, <>  | to strings and are applied in a case sensitive manner.  | 
  | |  <=, >=  | In the following example x, y and z are of type string. | 
  | |  < , >   | (eg: not((x <= 'AbC') and ('1x2y3z' <> y)) or (z == x)  | 
  | +----------+---------------------------------------------------------+ 
  | | in       | True only if x is a substring of y.                     | 
  | |          | (eg: x in y or 'abc' in 'abcdefgh')                     | 
  | +----------+---------------------------------------------------------+ 
  | | like     | True only if the string x matches the pattern y.        | 
  | |          | Available wildcard characters are '*' and '?' denoting  | 
  | |          | zero or more and zero or one matches respectively.      | 
  | |          | (eg: x like y or 'abcdefgh' like 'a?d*h')               | 
  | +----------+---------------------------------------------------------+ 
  | | ilike    | True only if the string x matches the pattern y in a    | 
  | |          | case insensitive manner. Available wildcard characters  | 
  | |          | are '*' and '?' denoting zero or more and zero or one   | 
  | |          | matches respectively.                                   | 
  | |          | (eg: x ilike y or 'a1B2c3D4e5F6g7H' ilike 'a?d*h')      | 
  | +----------+---------------------------------------------------------+ 
  | | [r0:r1]  | The closed interval[r0,r1] of the specified string.     | 
  | |          | eg: Given a string x with a value of 'abcdefgh' then:   | 
  | |          | 1. x[1:4] == 'bcde'                                     | 
  | |          | 2. x[ :4] == x[:8 / 2] == 'abcde'                       | 
  | |          | 3. x[2 + 1: ] == x[3:] =='defgh'                        | 
  | |          | 4. x[ : ] == x[:] == 'abcdefgh'                         | 
  | |          | 5. x[4/2:3+1] == x[2:4] == 'cde'                        | 
  | |          |                                                         | 
  | |          | Note: Both r0 and r1 are assumed to be integers, where  | 
  | |          | r0 <= r1. They may also be the result of an expression, | 
  | |          | in the event they have fractional components truncation | 
  | |          | shall be performed. (eg: 1.67 --> 1)                    | 
  | +----------+---------------------------------------------------------+ 
  | |  :=      | Assign the value of x to y. Where y is a mutable string | 
  | |          | or string range and x is either a string or a string    | 
  | |          | range. eg:                                              | 
  | |          | 1. y := x                                               | 
  | |          | 2. y := 'abc'                                           | 
  | |          | 3. y := x[:i + j]                                       | 
  | |          | 4. y := '0123456789'[2:7]                               | 
  | |          | 5. y := '0123456789'[2i + 1:7]                          | 
  | |          | 6. y := (x := '0123456789'[2:7])                        | 
  | |          | 7. y[i:j] := x                                          | 
  | |          | 8. y[i:j] := (x + 'abcdefg'[8 / 4:5])[m:n]              | 
  | |          |                                                         | 
  | |          | Note: For options 7 and 8 the shorter of the two ranges | 
  | |          | will denote the number characters that are to be copied.| 
  | +----------+---------------------------------------------------------+ 
  | |  +       | Concatenation of x and y. Where x and y are strings or  | 
  | |          | string ranges. eg                                       | 
  | |          | 1. x + y                                                | 
  | |          | 2. x + 'abc'                                            | 
  | |          | 3. x + y[:i + j]                                        | 
  | |          | 4. x[i:j] + y[2:3] + '0123456789'[2:7]                  | 
  | |          | 5. 'abc' + x + y                                        | 
  | |          | 6. 'abc' + '1234567'                                    | 
  | |          | 7. (x + 'a1B2c3D4' + y)[i:2j]                           | 
  | +----------+---------------------------------------------------------+ 
  | |  +=      | Append to x the value of y. Where x is a mutable string | 
  | |          | and y is either a string or a string range. eg:         | 
  | |          | 1. x += y                                               | 
  | |          | 2. x += 'abc'                                           | 
  | |          | 3. x += y[:i + j] + 'abc'                               | 
  | |          | 4. x += '0123456789'[2:7]                               | 
  | +----------+---------------------------------------------------------+ 
  | | <=>      | Swap the values of x and y. Where x and y are mutable   | 
  | |          | strings.  (eg: x <=> y)                                 | 
  | +----------+---------------------------------------------------------+ 
  | | []       | The string size operator returns the size of the string | 
  | |          | being actioned.                                         | 
  | |          | eg:                                                     | 
  | |          | 1. 'abc'[] == 3                                         | 
  | |          | 2. var max_str_length := max(s0[], s1[], s2[], s3[])    | 
  | |          | 3. ('abc' + 'd')[] == 6                                 | 
  | |          | 4. (('abc' + 'xyz')[1:4])[] == 4                        | 
  | +----------+---------------------------------------------------------+ 
  |  
  | (6) Control Structures 
  | +----------+---------------------------------------------------------+ 
  | |STRUCTURE | DEFINITION                                              | 
  | +----------+---------------------------------------------------------+ 
  | | if       | If x is true then return y else return z.               | 
  | |          | eg:                                                     | 
  | |          | 1. if (x, y, z)                                         | 
  | |          | 2. if ((x + 1) > 2y, z + 1, w / v)                      | 
  | |          | 3. if (x > y) z;                                        | 
  | |          | 4. if (x <= 2*y) { z + w };                             | 
  | +----------+---------------------------------------------------------+ 
  | | if-else  | The if-else/else-if statement. Subject to the condition | 
  | |          | branch the statement will return either the value of the| 
  | |          | consequent or the alternative branch.                   | 
  | |          | eg:                                                     | 
  | |          | 1. if (x > y) z; else w;                                | 
  | |          | 2. if (x > y) z; else if (w != u) v;                    | 
  | |          | 3. if (x < y) { z; w + 1; } else u;                     | 
  | |          | 4. if ((x != y) and (z > w))                            | 
  | |          |    {                                                    | 
  | |          |      y := sin(x) / u;                                   | 
  | |          |      z := w + 1;                                        | 
  | |          |    }                                                    | 
  | |          |    else if (x > (z + 1))                                | 
  | |          |    {                                                    | 
  | |          |      w := abs (x - y) + z;                              | 
  | |          |      u := (x + 1) > 2y ? 2u : 3u;                       | 
  | |          |    }                                                    | 
  | +----------+---------------------------------------------------------+ 
  | | switch   | The first true case condition that is encountered will  | 
  | |          | determine the result of the switch. If none of the case | 
  | |          | conditions hold true, the default action is assumed as  | 
  | |          | the final return value. This is sometimes also known as | 
  | |          | a multi-way branch mechanism.                           | 
  | |          | eg:                                                     | 
  | |          | switch                                                  | 
  | |          | {                                                       | 
  | |          |    case x > (y + z) : 2 * x / abs(y - z);               | 
  | |          |    case x < 3       : sin(x + y);                       | 
  | |          |    default          : 1 + x;                            | 
  | |          | }                                                       | 
  | +----------+---------------------------------------------------------+ 
  | | while    | The structure will repeatedly evaluate the internal     | 
  | |          | statement(s) 'while' the condition is true. The final   | 
  | |          | statement in the final iteration shall be used as the   | 
  | |          | return value of the loop.                               | 
  | |          | eg:                                                     | 
  | |          | while ((x -= 1) > 0)                                    | 
  | |          | {                                                       | 
  | |          |    y := x + z;                                          | 
  | |          |    w := u + y;                                          | 
  | |          | }                                                       | 
  | +----------+---------------------------------------------------------+ 
  | | repeat/  | The structure will repeatedly evaluate the internal     | 
  | | until    | statement(s) 'until' the condition is true. The final   | 
  | |          | statement in the final iteration shall be used as the   | 
  | |          | return value of the loop.                               | 
  | |          | eg:                                                     | 
  | |          | repeat                                                  | 
  | |          |    y := x + z;                                          | 
  | |          |    w := u + y;                                          | 
  | |          | until ((x += 1) > 100)                                  | 
  | +----------+---------------------------------------------------------+ 
  | | for      | The structure will repeatedly evaluate the internal     | 
  | |          | statement(s) while the condition is true. On each loop  | 
  | |          | iteration, an 'incrementing' expression is evaluated.   | 
  | |          | The conditional is mandatory whereas the initialiser    | 
  | |          | and incrementing expressions are optional.              | 
  | |          | eg:                                                     | 
  | |          | for (var x := 0; (x < n) and (x != y); x += 1)          | 
  | |          | {                                                       | 
  | |          |    y := y + x / 2 - z;                                  | 
  | |          |    w := u + y;                                          | 
  | |          | }                                                       | 
  | +----------+---------------------------------------------------------+ 
  | | break    | Break terminates the execution of the nearest enclosed  | 
  | | break[]  | loop, allowing for the execution to continue on external| 
  | |          | to the loop. The default break statement will set the   | 
  | |          | return value of the loop to NaN, where as the return    | 
  | |          | based form will set the value to that of the break      | 
  | |          | expression.                                             | 
  | |          | eg:                                                     | 
  | |          | while ((i += 1) < 10)                                   | 
  | |          | {                                                       | 
  | |          |    if (i < 5)                                           | 
  | |          |       j -= i + 2;                                       | 
  | |          |    else if (i % 2 == 0)                                 | 
  | |          |       break;                                            | 
  | |          |    else                                                 | 
  | |          |       break[2i + 3];                                    | 
  | |          | }                                                       | 
  | +----------+---------------------------------------------------------+ 
  | | continue | Continue results in the remaining portion of the nearest| 
  | |          | enclosing loop body to be skipped.                      | 
  | |          | eg:                                                     | 
  | |          | for (var i := 0; i < 10; i += 1)                        | 
  | |          | {                                                       | 
  | |          |    if (i < 5)                                           | 
  | |          |       continue;                                         | 
  | |          |    j -= i + 2;                                          | 
  | |          | }                                                       | 
  | +----------+---------------------------------------------------------+ 
  | | return   | Return immediately from within the current expression.  | 
  | |          | With the option of passing back a variable number of    | 
  | |          | values (scalar, vector or string). eg:                  | 
  | |          | 1. return [1];                                          | 
  | |          | 2. return [x, 'abx'];                                   | 
  | |          | 3. return [x, x + y,'abx'];                             | 
  | |          | 4. return [];                                           | 
  | |          | 5. if (x < y)                                           | 
  | |          |     return [x, x - y, 'result-set1', 123.456];          | 
  | |          |    else                                                 | 
  | |          |     return [y, x + y, 'result-set2'];                   | 
  | +----------+---------------------------------------------------------+ 
  | | ?:       | Ternary conditional statement, similar to that of the   | 
  | |          | above denoted if-statement.                             | 
  | |          | eg:                                                     | 
  | |          | 1. x ? y : z                                            | 
  | |          | 2. x + 1 > 2y ? z + 1 : (w / v)                         | 
  | |          | 3. min(x,y) > z ? (x < y + 1) ? x : y : (w * v)         | 
  | +----------+---------------------------------------------------------+ 
  | | ~        | Evaluate each sub-expression, then return as the result | 
  | |          | the value of the last sub-expression. This is sometimes | 
  | |          | known as multiple sequence point evaluation.            | 
  | |          | eg:                                                     | 
  | |          | ~(i := x + 1, j := y / z, k := sin(w/u)) == (sin(w/u))) | 
  | |          | ~{i := x + 1; j := y / z; k := sin(w/u)} == (sin(w/u))) | 
  | +----------+---------------------------------------------------------+ 
  | | [*]      | Evaluate any consequent for which its case statement is | 
  | |          | true. The return value will be either zero or the result| 
  | |          | of the last consequent to have been evaluated.          | 
  | |          | eg:                                                     | 
  | |          | [*]                                                     | 
  | |          | {                                                       | 
  | |          |   case (x + 1) > (y - 2)    : x := z / 2 + sin(y / pi); | 
  | |          |   case (x + 2) < abs(y + 3) : w / 4 + min(5y,9);        | 
  | |          |   case (x + 3) == (y * 4)   : y := abs(z / 6) + 7y;     | 
  | |          | }                                                       | 
  | +----------+---------------------------------------------------------+ 
  | | []       | The vector size operator returns the size of the vector | 
  | |          | being actioned.                                         | 
  | |          | eg:                                                     | 
  | |          | 1. v[]                                                  | 
  | |          | 2. max_size := max(v0[],v1[],v2[],v3[])                 | 
  | +----------+---------------------------------------------------------+ 
  |  
  | Note01: In  the  tables above, the symbols x, y, z, w, u  and v  where 
  | appropriate may represent any of one the following: 
  |  
  |    1. Literal numeric/string value 
  |    2. A variable 
  |    3. A vector element 
  |    4. A vector 
  |    5. A string 
  |    6. An expression comprised of [1], [2] or [3] (eg: 2 + x / vec[3]) 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 09 - FUNDAMENTAL TYPES] 
  | ExprTk supports three fundamental types which can be used freely in 
  | expressions. The types are as follows: 
  |  
  |    (1) Scalar 
  |    (2) Vector 
  |    (3) String 
  |  
  |  
  | (1) Scalar Type 
  | The scalar type  is a singular  numeric value. The  underlying type is 
  | that used  to specialise  the ExprTk  components (float,  double, long 
  | double, MPFR et al). 
  |  
  |  
  | (2) Vector Type 
  | The vector type is a fixed size sequence of contiguous scalar  values. 
  | A  vector  can be  indexed  resulting in  a  scalar value.  Operations 
  | between a vector and scalar will result in a vector with a size  equal 
  | to that  of the  original vector,  whereas operations  between vectors 
  | will result in a  vector of size equal  to that of the  smaller of the 
  | two. In both mentioned cases, the operations will occur element-wise. 
  |  
  |  
  | (3) String Type 
  | The string type is a variable length sequence of 8-bit chars.  Strings 
  | can be  assigned and  concatenated to  one another,  they can  also be 
  | manipulated via sub-ranges using the range definition syntax.  Strings 
  | however can not interact with scalar or vector types. 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 10 - COMPONENTS] 
  | There are three primary components, that are specialised upon a  given 
  | numeric type, which make up the core of ExprTk. The components are  as 
  | follows: 
  |  
  |    (1) Symbol Table  exprtk::symbol_table<NumericType> 
  |    (2) Expression    exprtk::expression<NumericType> 
  |    (3) Parser        exprtk::parser<NumericType> 
  |  
  |  
  | (1) Symbol Table 
  | A structure that is used  to store references to variables,  constants 
  | and functions that are to  be used within expressions. Furthermore  in 
  | the context  of composited  recursive functions  the symbol  table can 
  | also be thought of as a simple representation of a stack specific  for 
  | the expression(s) that reference it. The following is a list of the 
  | types a symbol table can handle: 
  |  
  |    (a) Numeric variables 
  |    (b) Numeric constants 
  |    (c) Numeric vector elements 
  |    (d) String variables 
  |    (e) String constants 
  |    (f) Functions 
  |    (g) Vararg functions 
  |  
  |  
  | During the compilation  process if an  expression is found  to require 
  | any  of  the  elements   noted  above,  the  expression's   associated 
  | symbol_table  will  be  queried  for  the  element  and  if  present a 
  | reference to the element will be embedded within the expression's AST. 
  | This allows for the original  element to be modified independently  of 
  | the  expression  instance  and  to also  allow  the  expression  to be 
  | evaluated using the current value of the element. 
  |  
  | The  example  below demonstrates  the  relationship between variables, 
  | symbol_table and expression. Note  the variables are modified  as they 
  | normally would in a program, and when the expression is  evaluated the 
  | current values assigned to the variables shall be used. 
  |  
  |    typedef exprtk::symbol_table<double> symbol_table_t; 
  |    typedef exprtk::expression<double>   expression_t; 
  |    typedef exprtk::parser<double>       parser_t; 
  |  
  |    double x = 0; 
  |    double y = 0; 
  |  
  |    symbol_table_t symbol_table; 
  |    expression_t   expression; 
  |    parser_t       parser; 
  |  
  |    std::string expression_string = "x * y + 3" 
  |  
  |    symbol_table.add_variable("x",x); 
  |    symbol_table.add_variable("y",y); 
  |  
  |    expression.register_symbol_table(symbol_table); 
  |  
  |    parser.compile(expression_string,expression); 
  |  
  |    x = 1.0; 
  |    y = 2.0; 
  |    expression.value(); // 1 * 2 + 3 
  |  
  |    x = 3.7; 
  |    expression.value(); // 3.7 * 2 + 3 
  |  
  |    y = -9.0; 
  |    expression.value(); // 3.7 * -9 + 3 
  |  
  |    // 'x * -9 + 3' for x in range of [0,100) in steps of 0.0001 
  |    for (x = 0.0; x < 100.0; x += 0.0001) 
  |    { 
  |       expression.value(); // x * -9 + 3 
  |    } 
  |  
  |  
  | Note02: Any  variable  reference  provided  to  a  given  symbol_table 
  | instance, must have a lifetime at least as long as the lifetime of the 
  | symbol_table  instance.  In  the  event  the  variable  reference   is 
  | invalidated  before  the  symbol_table  or  any  dependent  expression 
  | instances  have  been  destructed,  then  any  associated   expression 
  | evaluations or variable referencing via the symbol_table instance will 
  | result in undefined behaviour. 
  |  
  | The following bit of  code instantiates a symbol_table  and expression 
  | instance,  then  proceeds  to   demonstrate  various  ways  in   which 
  | references to  variables can  be added  to the  symbol_table, and  how 
  | those  references are  subsequently invalidated  resulting in  various 
  | forms of undefined behaviour. 
  |  
  |    typedef exprtk::symbol_table<double> symbol_table_t; 
  |    typedef exprtk::expression<double>   expression_t; 
  |  
  |    symbol_table_t symbol_table; 
  |    expression_t   expression; 
  |  
  |    std::deque<double > y {1.1, 2.2, 3.3}; 
  |    std::vector<double> z {4.4, 5.5, 6.6}; 
  |    double* w = new double(123.456); 
  |  
  |    { 
  |       double x = 123.4567; 
  |       symbol_table.add_variable("x", x); 
  |    }             // Reference to variable x has been invalidated 
  |  
  |  
  |    symbol_table.add_variable("y", y.back()); 
  |  
  |    y.pop_back(); // Reference to variable y has been invalidated 
  |  
  |  
  |    symbol_table.add_variable("z", z.front()); 
  |  
  |    z.erase(z.begin()); 
  |                  // Reference to variable z has been invalidated 
  |  
  |    symbol_table.add_variable("w", *w); 
  |  
  |    delete w;     // Reference to variable w has been invalidated 
  |  
  |    const std::string expression_string = "x + y / z * w" 
  |  
  |                  // Compilation of expression will succeed 
  |    parser.compile(expression_string,expression); 
  |  
  |    expression.value(); 
  |                  // Evaluation will result in undefined behaviour 
  |                  // due to 'x' and 'w' having been destroyed. 
  |  
  |    symbol_table.get_variable("x")->ref() = 135.791; 
  |                  // Assignment will result in undefined behaviour 
  |  
  |  
  | A compiled expression that references variables from a symbol_table is 
  | dependent on  that symbol_table  instance and  the variables  it holds 
  | being valid. 
  |  
  |    typedef exprtk::symbol_table<double> symbol_table_t; 
  |    typedef exprtk::expression<double>   expression_t; 
  |  
  |    symbol_table_t symbol_table; 
  |    expression_t   expression; 
  |  
  |    double x = 123.456; 
  |  
  |    symbol_table.add_variable("x", x); 
  |  
  |    const std::string expression_string = "(x + 1) / 2" 
  |  
  |                  // Compilation of the expression will succeed 
  |    parser.compile(expression_string,expression); 
  |  
  |                  // Clear all variables from symbol_table 
  |    symbol_table.clear(); 
  |  
  |    expression.value(); 
  |                  // Evaluation will result in undefined behaviour 
  |                  // because the reference to 'x' having been destroyed 
  |                  // during the clearing of the symbol_table 
  |  
  |  
  | In  the  above  example, an  expression  is  compiled that  references 
  | variable "x". As part of the compilation process the node holding  the 
  | variable "x" is obtained from the symbol_table and embedded in the AST 
  | of the  expression -  in short  the expression  is now referencing the 
  | node that holds  the variable "x".  The following diagram  depicts the 
  | dependencies between  the  variable   x,  the  symbol  table  and  the 
  | expression: 
  |  
  |       +--[Symbol Table]--+ 
  |       |                  | 
  |       | +- ------+       | 
  |       | | x-node |       | 
  |       | +-----A--+       |    +--[Expression]--+ 
  |       +---|---|----------+    |  +---------+   | 
  |           v   |               |  |  A.S.T  |   | 
  |           |   +--------<--------[.]        |   | 
  |     +-----+                   |  +---------+   | 
  |     |                         +----------------+ 
  |   +-v-[variable]---+ 
  |   | x: 123.456     | 
  |   +----------------+ 
  |  
  |  
  | When the  clear method  is called  on the  symbol table  the X-Node is 
  | destroyed, so now the expression  is referencing a node that  has been 
  | destroyed.  From  this point  onwards  any attempts  to  reference the 
  | expression instance will result in undefined behaviour. Simply put the 
  | above  example  violates  the requirement  that  the  lifetime of  any 
  | objects referenced by  expressions should exceed  the lifetime of  the 
  | expression instance. 
  |  
  |    typedef exprtk::symbol_table<double> symbol_table_t; 
  |    typedef exprtk::expression<double>   expression_t; 
  |  
  |    symbol_table_t symbol_table; 
  |    expression_t   expression; 
  |  
  |    double x = 123.456; 
  |  
  |    symbol_table.add_variable("x", x); 
  |  
  |    const std::string expression_string = "(x + 1) / 2" 
  |  
  |                  // Compilation of the expression will succeed 
  |    parser.compile(expression_string,expression); 
  |  
  |    expression.value(); 
  |  
  |                  // Release the expression and its dependents 
  |    expression.release(); 
  |  
  |                  // Clear all variables from symbol_table 
  |    symbol_table.clear(); 
  |  
  |    expression.value(); 
  |                  // Will return null_node value of NaN 
  |  
  |  
  | In the above example the expression is released before the  associated 
  | symbol_table is cleared of its variables, which resolves the undefined 
  | behaviour issue noted in the previous example. 
  |  
  | Note03: It  is possible  to  register  multiple  symbol_tables  with a 
  | single  expression object.  In the  event an  expression has  multiple 
  | symbol tables, and where  there exists conflicts between  symbols, the 
  | compilation stage  will resolve  the conflicts  based on  the order of 
  | registration  of  the  symbol_tables to  the  expression.  For a  more 
  | expansive discussion please review section [17 - Hierarchies Of Symbol 
  | Tables] 
  |  
  |    typedef exprtk::symbol_table<double> symbol_table_t; 
  |    typedef exprtk::expression<double>   expression_t; 
  |    typedef exprtk::parser<double>       parser_t; 
  |  
  |    symbol_table_t symbol_table0; 
  |    symbol_table_t symbol_table1; 
  |  
  |    expression_t   expression; 
  |    parser_t       parser; 
  |  
  |    double x0 = 123.0; 
  |    double x1 = 678.0; 
  |  
  |    std::string expression_string = "x + 1" 
  |  
  |    symbol_table0.add_variable("x",x0); 
  |    symbol_table1.add_variable("x",x1); 
  |  
  |    expression.register_symbol_table(symbol_table0); 
  |    expression.register_symbol_table(symbol_table1); 
  |  
  |    parser.compile(expression_string,expression); 
  |  
  |    expression.value(); // 123 + 1 
  |  
  |  
  | The symbol table supports  adding references to external  instances of 
  | types  that  can  be accessed  within  expressions  via the  following 
  | methods: 
  |  
  |   1. bool add_variable    (const std::string& name, scalar_t&        ) 
  |   2. bool add_constant    (const std::string& name, const scalar_t&  ) 
  |   3. bool add_stringvar   (const std::string& name, std::string&     ) 
  |   4. bool add_vector      (const std::string& name, vector_type&     ) 
  |   5. bool add_function    (const std::string& name, function_t&      ) 
  |   6. bool create_stringvar(const std::string& name,const std::string&) 
  |   7. bool create_variable (const std::string& name, const T&         ) 
  |  
  |  
  | Note04: The 'vector' type must be comprised from a contiguous array of 
  | scalars with a size that is  larger than zero. The vector type  itself 
  | can be any one of the following: 
  |  
  |    1. std::vector<scalar_t> 
  |    2. scalar_t(&v)[N] 
  |    3. scalar_t* and array size 
  |    4. exprtk::vector_view<scalar_t> 
  |  
  |  
  | When  registering  a variable,  vector,  string or  function  with  an 
  | instance of a symbol_table, the call to 'add_...' may fail and  return 
  | a false result due to one or more of the following reasons: 
  |  
  |   1. Variable name contains invalid characters or is ill-formed 
  |   2. Variable name conflicts with a reserved word (eg: 'while') 
  |   3. Variable name conflicts with a previously registered variable 
  |   4. A vector of size (length) zero is being registered 
  |   5. A free function exceeding fifteen parameters is being registered 
  |   6. The symbol_table instance is in an invalid state 
  |  
  |  
  | Note05: The symbol_table has a method called clear, which when invoked 
  | will clear  all variables,  vectors, strings  and functions registered 
  | with the symbol_table instance. If  this method is to be  called, then 
  | one  must  make  sure  that  all  compiled  expression  instances that 
  | reference  variables  belonging  to  that  symbol_table  instance  are 
  | released (aka call  release method on  expression) before calling  the 
  | clear  method  on  the  symbol_table  instance,  otherwise   undefined 
  | behaviours will occur. 
  |  
  | A further property of symbol tables is that they can be classified  at 
  | instantiation as either being mutable (by default) or immutable.  This 
  | property determines if variables,  vectors or strings registered  with 
  | the symbol  table can  undergo modifications  within expressions  that 
  | reference  them.  The   following  demonstrates  construction   of  an 
  | immutable symbol table instance: 
  |  
  |   symbol_table_t immutable_symbol_table 
  |      (symbol_table_t::symtab_mutability_type::e_immutable); 
  |  
  |  
  | When a symbol table, that has been constructed as  being immutable, is 
  | registered with an expression, any statements in the expression string 
  | that modify  the variables  that are  managed by  the immutable symbol 
  | table will result in a compilation error. The operations that  trigger 
  | the mutability constraint are the following assignment operators: 
  |  
  |   1. Assignment:       := 
  |   2. Assign operation: +=, -=, *=, /= , %= 
  |  
  |    const std::string expression_str = "x += x + 123.456" 
  |  
  |    symbol_table_t immutable_symbol_table 
  |       (symbol_table_t::symtab_mutability_type::e_immutable); 
  |  
  |    T x = 0.0; 
  |  
  |    immutable_symbol_table.add_variable("x" , x); 
  |  
  |    expression_t expression; 
  |    expression.register_symbol_table(immutable_symbol_table); 
  |  
  |    parser_t parser; 
  |  
  |    parser.compile(expression_str, expression); 
  |       // Compile error because of assignment to variable x 
  |  
  |  
  | In the above example, variable x is registered to an immutable  symbol 
  | table,  making it  an immutable  variable within  the context  of any 
  | expressions that  reference it.  The expression  string being compiled 
  | uses the addition assignment operator  which will modify the value  of 
  | variable x.  The compilation  process detects  this semantic violation 
  | and proceeds to halt compilation and return the appropriate error. 
  |  
  | One of the main reasons for  this functionality is that, one may  want 
  | the immutability  properties that  come with  constness of  a variable 
  | such  as  scalars,  vectors  and  strings,  but  not  necessarily  the 
  | accompanying  compile  time  const-folding  optimisations,  that would 
  | result in  the value  of the  variables being  retrieved only  once at 
  | compile time, causing external updates to the variables to not be part 
  | of the expression evaluation. 
  |  
  |    symbol_table_t immutable_symbol_table 
  |       (symbol_table_t::symtab_mutability_type::e_immutable); 
  |  
  |    T x = 0.0; 
  |  
  |    const std::string expression_str = "x + (y + y)" 
  |  
  |    immutable_symbol_table.add_variable("x" , x    ); 
  |    immutable_symbol_table.add_constant("y" , 123.0); 
  |  
  |    expression_t expression; 
  |    expression.register_symbol_table(immutable_symbol_table); 
  |  
  |    parser_t parser; 
  |    parser.compile(expression_str, expression); 
  |  
  |    for (; x < 10.0; ++x) 
  |    { 
  |       const auto expected_value = x + (123.0 + 123.0); 
  |       const auto result_value   = expression.value(); 
  |       assert(expression.value() !=  expected_value); 
  |    } 
  |  
  |  
  | In the above example,  there are two variables  X and Y. Where  Y is a 
  | constant and X is a normal variable. Both are registered with a symbol 
  | table that is immutable. The  expression when compiled will result  in 
  | the "(y + y)" part being  const-folded at compile time to the  literal 
  | value of 246. Whereas  the current value of  X, being updated via  the 
  | for-loop, externally to  the expression and  the symbol table shall be 
  | observable to the expression upon each evaluation. 
  |  
  |  
  | (2) Expression 
  | A structure that holds an Abstract Syntax Tree or AST for a  specified 
  | expression and is used to evaluate said expression. Evaluation of  the 
  | expression is accomplished by performing a post-order traversal of the 
  | AST.  If  a  compiled  Expression  uses  variables  or  user   defined 
  | functions, it will have an associated Symbol Table, which will contain 
  | references  to said  variables, functions  or strings. An example  AST 
  | structure for the denoted expression is as follows: 
  |  
  | Expression:  z := (x + y^-2.345) * sin(pi / min(w - 7.3,v)) 
  |  
  |                   [Root] 
  |                     | 
  |                [Assignment] 
  |         ________/        \_____ 
  |        /                       \ 
  |  Variable(z)            [Multiplication] 
  |                 ____________/      \___________ 
  |                /                               \ 
  |               /                      [Unary-Function(sin)] 
  |          [Addition]                            | 
  |       ____/      \____                    [Division] 
  |      /                \                 ___/      \___ 
  |  Variable(x)   [Exponentiation]        /              \ 
  |               ______/   \______  Constant(pi) [Binary-Function(min)] 
  |              /                 \                ____/    \___ 
  |         Variable(y)        [Negation]          /             \ 
  |                                |              /          Variable(v) 
  |                         Constant(2.345)      / 
  |                                             / 
  |                                      [Subtraction] 
  |                                    ____/      \____ 
  |                                   /                \ 
  |                              Variable(w)      Constant(7.3) 
  |  
  |  
  | The above denoted AST shall be evaluated in the following order: 
  |  
  |    (01) Load Variable  (z)        (10) Load Constant  (7.3) 
  |    (02) Load Variable  (x)        (11) Subtraction    (09 & 10) 
  |    (03) Load Variable  (y)        (12) Load Variable  (v) 
  |    (04) Load Constant  (2.345)    (13) Min            (11 & 12) 
  |    (05) Negation       (04)       (14) Division       (08 & 13) 
  |    (06) Exponentiation (03 & 05)  (15) Sin            (14) 
  |    (07) Addition       (02 & 06)  (16) Multiplication (07 & 15) 
  |    (08) Load Constant  (pi)       (17) Assignment     (01 & 16) 
  |    (09) Load Variable  (w) 
  |  
  |  
  | Generally an expression in ExprTk can be thought of as a free function 
  | similar to those  found in imperative  languages. This form  of pseudo 
  | function will have a name, it may have a set of one or more inputs and 
  | will return at least one value as its result. Furthermore the function 
  | when invoked, may  cause a side-effect  that changes the  state of the 
  | host program. 
  |  
  | As an  example the  following is  a pseudo-code  definition of  a free 
  | function that performs a computation taking four inputs, modifying one 
  | of them and returning a value based on some arbitrary calculation: 
  |  
  |    ResultType foo(InputType x, InputType y, InputType z, InputType w) 
  |    { 
  |       w = 2 * x^y + z;        // Side-Effect 
  |       return abs(x - y) / z;  // Return Result 
  |    } 
  |  
  |  
  | Given the above definition the following is a functionally  equivalent 
  | version using ExprTk: 
  |  
  |    const std::string foo_str = 
  |       " w := 2 * x^y + z; " 
  |       " abs(x - y) / z;   " 
  |  
  |    T x, y, z, w; 
  |  
  |    symbol_table_t symbol_table; 
  |    symbol_table.add_variable("x",x); 
  |    symbol_table.add_variable("y",y); 
  |    symbol_table.add_variable("z",z); 
  |    symbol_table.add_variable("w",w); 
  |  
  |    expression_t foo; 
  |    foo.register_symbol_table(symbol_table); 
  |  
  |    parser_t parser; 
  |    if (!parser.compile(foo_str,foo)) 
  |    { 
  |       // Error in expression... 
  |       return; 
  |    } 
  |  
  |    T result = foo.value(); 
  |  
  |  
  | (3) Parser 
  | A  component  which  takes  as input  a  string  representation  of an 
  | expression and attempts to compile said input with the result being an 
  | instance  of  Expression.  If  an  error  is  encountered  during  the 
  | compilation  process, the  parser will  stop compiling  and return  an 
  | error status code,  with a more  detailed description of  the error(s) 
  | and  its  location  within  the  input  provided  by  the  'get_error' 
  | interface. 
  |  
  | Note06: The exprtk::expression and exprtk::symbol_table components are 
  | reference counted entities. Copy constructing or assigning to or  from 
  | either component will result in  a shallow copy and a  reference count 
  | increment,  rather  than  a  complete  replication.  Furthermore   the 
  | expression  and symbol_table  components being  Default-Constructible, 
  | Copy-Constructible  and  Copy-Assignable  make  them  compatible  with 
  | various  C++  standard  library   containers  and  adaptors  such   as 
  | std::vector, std::map, std::stack etc. 
  |  
  | The following is  an example of  two unique expressions,  after having 
  | been instantiated and  compiled,  one expression  is  assigned to  the 
  | other. The diagrams depict  their initial and post  assignment states, 
  | including  which  control  block each  expression references and their 
  | associated reference counts. 
  |  
  |  
  |     exprtk::expression e0; // constructed expression, eg: x + 1 
  |     exprtk::expression e1; // constructed expression, eg: 2z + y 
  |  
  |   +-----[ e0 cntrl block]----+     +-----[ e1 cntrl block]-----+ 
  |   | 1. Expression Node 'x+1' |     | 1. Expression Node '2z+y' | 
  |   | 2. Ref Count: 1          |<-+  | 2. Ref Count: 1           |<-+ 
  |   +--------------------------+  |  +---------------------------+  | 
  |                                 |                                 | 
  |     +--[ e0 expression]--+      |    +--[ e1 expression]--+       | 
  |     | 1. Reference to    ]------+    | 1. Reference to    ]-------+ 
  |     | e0 Control Block   |           | e1 Control Block   | 
  |     +--------------------+           +--------------------+ 
  |  
  |  
  |    e0 = e1; // e0 and e1 are now 2z+y 
  |  
  |                +-----[ e1 cntrl block]-----+ 
  |                | 1. Expression Node '2z+y' | 
  |   +----------->| 2. Ref Count: 2           |<----------+ 
  |   |            +---------------------------+           | 
  |   |                                                    | 
  |   |   +--[ e0 expression]--+  +--[ e1 expression]--+   | 
  |   +---[ 1. Reference to    |  | 1. Reference to    ]---+ 
  |       | e1 Control Block   |  | e1 Control Block   | 
  |       +--------------------+  +--------------------+ 
  |  
  | The reason for  the above complexity  and restrictions of  deep copies 
  | for the expression and symbol_table components is because  expressions 
  | may include user defined variables or functions. These are embedded as 
  | references into the expression's AST. When copying an expression, said 
  | references  need to  also  be  copied. If  the references  are blindly 
  | copied,  it  will then  result  in two  or more  identical expressions 
  | utilising the exact same  references for variables. This  obviously is 
  | not the  default assumed  scenario and  will give  rise to non-obvious 
  | behaviours  when  using the  expressions in  various contexts such  as 
  | multi-threading et al. 
  |  
  | The prescribed method for cloning an expression is to compile it  from 
  | its string  form. Doing so will allow the 'user' to  properly consider 
  | the exact source of user defined variables and functions. 
  |  
  | Note07: The exprtk::parser  is  a  non-copyable  and  non-thread  safe 
  | component, and should only be shared via either a reference, a  shared 
  | pointer  or  a  std::ref  mechanism,  and  considerations  relating to 
  | synchronisation  taken  into  account  where  appropriate.  The parser 
  | represents an object factory,  specifically a factory of  expressions, 
  | and generally should  not be instantiated  solely on a  per expression 
  | compilation basis. 
  |  
  | The  following  diagram  and  example depicts  the  flow  of  data and 
  | operations  for  compiling  multiple expressions  via  the  parser and 
  | inserting  the  newly  minted  exprtk::expression  instances  into   a 
  | std::vector. 
  |  
  |                       +----[exprtk::parser]---+ 
  |                       |   Expression Factory  | 
  |                       | parser_t::compile(...)| 
  |                     +--> ~.~.~.~.~.~.~.~.~.~ ->--+ 
  |                     | +-----------------------+  | 
  |  Expressions in     |                            |  Expressions as 
  |  string form        ^                            V  exprtk::expression 
  |                     |                            |  instances 
  |  [s0:'x+1']--->--+  |                            |  +-[e0: x+1] 
  |                  |  |                            |  | 
  |  [s1:'2z+y']-->--+--+                            +->+-[e1: 2z+y] 
  |                  |                                  | 
  |  [s2:'sin(k+w)']-+                                  +-[e2: sin(k+w)] 
  |  
  |  
  |    const std::string expression_str[3] = 
  |       { 
  |          "x + 1", 
  |          "2x + y", 
  |          "sin(k + w)" 
  |       }; 
  |  
  |    std::vector<expression_t> expression_list; 
  |  
  |    parser_t       parser; 
  |    expression_t   expression; 
  |    symbol_table_t symbol_table; 
  |  
  |    expression.register_symbol_table(symbol_table); 
  |  
  |    for (std::size_t i = 0; i < 3; ++i) 
  |    { 
  |       if (parser.compile(expression_str[i],expression)) 
  |       { 
  |          expression_list.push_back(expression); 
  |       } 
  |       else 
  |         std::cout << "Error in " << expression_str[i] << "\n" 
  |    } 
  |  
  |    for (auto& e : expression_list) 
  |    { 
  |       e.value(); 
  |    } 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 11 - COMPILATION OPTIONS] 
  | The exprtk::parser  when being  instantiated takes  as input  a set of 
  | options  to be  used during  the compilation  process of  expressions. 
  | An  example instantiation  of exprtk::parser  where only  the  joiner, 
  | commutative and strength reduction options are enabled is as  follows: 
  |  
  |    typedef exprtk::parser<NumericType>::settings_t settings_t; 
  |  
  |    const std::size_t compile_options = 
  |                         settings_t::e_joiner            + 
  |                         settings_t::e_commutative_check + 
  |                         settings_t::e_strength_reduction; 
  |  
  |    parser_t parser(compile_options); 
  |  
  |  
  | Currently  nine  types  of compile  time  options  are supported,  and 
  | enabled by default. The options and their explanations are as follows: 
  |  
  |    (1) Replacer 
  |    (2) Joiner 
  |    (3) Numeric Check 
  |    (4) Bracket Check 
  |    (5) Sequence Check 
  |    (6) Commutative Check 
  |    (7) Strength Reduction Check 
  |    (8) Stack And Node Depth Check 
  |    (9) Expression Size Check 
  |  
  |  
  | (1) Replacer (e_replacer) 
  | Enable replacement of specific  tokens with other tokens.  For example 
  | the token  "true" of  type symbol  shall be  replaced with the numeric 
  | token of value one. 
  |  
  |    (a) (x < y) == true   --->  (x < y) == 1 
  |    (b) false == (x > y)  --->  0 == (x > y) 
  |  
  |  
  | (2) Joiner (e_joiner) 
  | Enable  joining  of  multi-character  operators  that  may  have  been 
  | incorrectly  disjoint in the  string  representation  of the specified 
  | expression. For example the consecutive tokens of ">" "=" will  become 
  | ">=" representing  the "greater  than or  equal to"  operator. If  not 
  | properly resolved the  original form will  cause a compilation  error. 
  | The  following  is  a listing  of the  scenarios that  the joiner  can 
  | handle: 
  |  
  |    (a) '>' '='  --->  '>='  (gte) 
  |    (b) '<' '='  --->  '<='  (lte) 
  |    (c) '=' '='  --->  '=='  (equal) 
  |    (d) '!' '='  --->  '!='  (not-equal) 
  |    (e) '<' '>'  --->  '<>'  (not-equal) 
  |    (f) ':' '='  --->  ':='  (assignment) 
  |    (g) '+' '='  --->  '+='  (addition assignment) 
  |    (h) '-' '='  --->  '-='  (subtraction assignment) 
  |    (i) '*' '='  --->  '*='  (multiplication assignment) 
  |    (j) '/' '='  --->  '/='  (division assignment) 
  |    (k) '%' '='  --->  '%='  (modulo assignment) 
  |    (l) '+' '-'  --->  '-'   (subtraction) 
  |    (m) '-' '+'  --->  '-'   (subtraction) 
  |    (n) '-' '-'  --->  '+'   (addition) 
  |    (o) '<=' '>' --->  '<=>' (swap) 
  |  
  |  
  | An example of the transformation that takes place is as follows: 
  |  
  |    (a) (x > = y) and (z ! = w)  --->  (x >= y) and (z != w) 
  |  
  |  
  | (3) Numeric Check (e_numeric_check) 
  | Enable validation of tokens representing numeric types so as to  catch 
  | any errors prior  to the costly  process of the  main compilation step 
  | commencing. 
  |  
  |  
  | (4) Bracket Check (e_bracket_check) 
  | Enable  the  check for  validating  the ordering  of  brackets in  the 
  | specified expression. 
  |  
  |  
  | (5) Sequence Check (e_sequence_check) 
  | Enable the  check for  validating that  sequences of  either pairs  or 
  | triplets of tokens make sense.  For example the following sequence  of 
  | tokens when encountered will raise an error: 
  |  
  |    (a) (x + * 3)  --->  sequence error 
  |  
  |  
  | (6) Commutative Check (e_commutative_check) 
  | Enable the check that will transform sequences of pairs of tokens that 
  | imply a multiplication operation.  The following are some  examples of 
  | such transformations: 
  |  
  |    (a) 2x             --->  2 * x 
  |    (b) 25x^3          --->  25 * x^3 
  |    (c) 3(x + 1)       --->  3 * (x + 1) 
  |    (d) (x + 1)4       --->  (x + 1) * 4 
  |    (e) 5foo(x,y)      --->  5 * foo(x,y) 
  |    (f) foo(x,y)6 + 1  --->  foo(x,y) * 6 + 1 
  |    (g) (4((2x)3))     --->  4 * ((2 * x) * 3) 
  |    (h) w / (x - y)z   --->  w / (x - y) * z 
  |  
  |  
  | (7) Strength Reduction Check (e_strength_reduction) 
  | Enable  the  use  of  strength  reduction  optimisations  during   the 
  | compilation  process.  In  ExprTk  strength  reduction   optimisations 
  | predominantly involve  transforming sub-expressions  into other  forms 
  | that  are algebraically  equivalent yet  less costly  to compute.  The 
  | following are examples of the various transformations that can occur: 
  |  
  |    (a) (x / y) / z               --->  x / (y * z) 
  |    (b) (x / y) / (z / w)         --->  (x * w) / (y * z) 
  |    (c) (2 * x) - (2 * y)         --->  2 * (x - y) 
  |    (d) (2 / x) / (3 / y)         --->  (2 / 3) / (x * y) 
  |    (e) (2 * x) * (3 * y)         --->  6 * (x * y) 
  |    (f) (2 * x) * (2 - 4 / 2)     --->  0 
  |    (g) (3 - 6 / 2) / (2 * x)     --->  0 
  |    (h) avg(x,y,z) * (2 - 4 / 2)  --->  0 
  |  
  |  
  | Note08: When using strength reduction in conjunction with  expressions 
  | whose inputs or sub-expressions may result in values nearing either of 
  | the bounds of the underlying  numeric type (eg: double), there  may be 
  | the  possibility of a decrease in the precision of results. 
  |  
  | In  the following  example the  given expression  which represents  an 
  | attempt at computing the average  between x and y will  be transformed 
  | as follows: 
  |  
  |    (0.5 * x) + (y * 0.5) ---> 0.5 * (x + y) 
  |  
  | There  may be  situations where  the above  transformation will  cause 
  | numerical overflows and  that the original  form of the  expression is 
  | desired over the strength reduced form. In these situations it is best 
  | to turn off strength reduction optimisations  or to use a type with  a 
  | larger numerical bound. 
  |  
  |  
  | (8) Stack And Node Depth Check 
  | ExprTk  incorporates   a  recursive   descent  parser.   When  parsing 
  | expressions comprising inner sub-expressions, the recursive nature  of 
  | the parsing process causes the stack to grow. If the expression causes 
  | the stack to grow  beyond the stack size  limit, this would lead  to a 
  | stackoverflow  and  its  associated  stack  corruption  and   security 
  | vulnerability issues. 
  |  
  | Similarly to parsing, evaluating an expression may cause the stack  to 
  | grow. Such things like user defined functions, composite functions and 
  | the general nature of the AST  being evaluated can cause the stack  to 
  | grow,  and may  result in  potential stackoverflow  issues as  denoted 
  | above. 
  |  
  | ExprTk provides a set of checks that prevent both of the above denoted 
  | problems at  compile time.  These checks rely on  two specific  limits 
  | being set on the parser settings instance, these limits are: 
  |  
  |    1. max_stack_depth (default: 400  ) 
  |    2. max_node_depth  (default: 10000) 
  |  
  |  
  | The following demonstrates how these two parser parameters can be set: 
  |  
  |    parser_t parser; 
  |  
  |    parser.settings().set_max_stack_depth(100); 
  |    parser.settings().set_max_node_depth(200); 
  |  
  |  
  | In  the  above  code, during  parsing if  the stack  depth reaches  or 
  | exceeds  100 levels,  the parsing  process will  immediately halt  and 
  | return with a failure.  Similarly, during synthesizing the  AST nodes, 
  | if the  compilation process  detects an  AST tree  depth exceeding 200 
  | levels the parsing process will halt and return a parsing failure. 
  |  
  |  
  | (9) Expression Size Check 
  | ExprTk allows for expression  local variables, vectors and  strings to 
  | be defined. As such the amount  of data in terms of bytes  consumed by 
  | the  expression  locals  can  also  be  limited,  so  as  to   prevent 
  | expressions once compiled from consuming large amounts of data. 
  |  
  | The maximum number of bytes an expression's locally defined  variables 
  | may use can  be set via  the parser settings  prior to compilation  as 
  | follows: 
  |  
  |    using expression_t = exprtk::expression<double>; 
  |    using parser_t     = exprtk::parser<double>; 
  |  
  |    expression_t expression; 
  |    parser_t     parser; 
  |  
  |    parser.settings().set_max_total_local_symbol_size_bytes(16); 
  |  
  |    const std::string expression1 = "var x := 1; var y := 1;" 
  |    const std::string expression2 = "var x := 1; var v[2] := [1];" 
  |  
  |    expression_t expression; 
  |  
  |    parser.compile(expression1, expression); // compilation success 
  |    parser.compile(expression2, expression); // compilation error 
  |  
  | In the above example, the expression's max total local symbol size has 
  | been set to 16 bytes. The numeric type being used is double which is 8 
  | bytes  per  instance.  The  first  expression  (expression1)  compiles 
  | successfully because the total local symbol size is 16 bytes, 8  bytes 
  | for each of the variables x and y. 
  |  
  | However the second expression (expression2) fails to compile. This  is 
  | because the  total number  of bytes  needed for  the expression  is 24 
  | bytes, 8 bytes for  the variable x and  16 bytes for the  vector v, as 
  | such  exceeding  the  previously  set limit  of  16  bytes.  The error 
  | diagnostic generated by  the parser on  compilation failure will  look 
  | like the following: 
  |  
  |    ERR161 - Adding vector 'v' of size 2 bytes will exceed max total 
  |    local symbol size of: 16 bytes, current total size: 8 bytes 
  |  
  |  
  | (10) Vector Size Check 
  | When  defining  an  expression local  vector,  ExprTk  uses a  default 
  | maximum vector size of two billion elements. One may want to limit the 
  | maximum vector size to be either smaller or larger than the  specified 
  | default value. The  maximum size value  can be changed  via the parser 
  | settings. 
  |  
  |    parser_t parser; 
  |  
  |    parser.settings().set_max_local_vector_size(1000000); 
  |  
  |    std::string expression1 = "var v[1e6] := [123]" 
  |    std::string expression2 = "var v[1e9] := [123]" 
  |  
  |    expression_t expression; 
  |  
  |    parser.compile(expression1, expression); // compilation success 
  |    parser.compile(expression2, expression); // compilation error 
  |  
  |  
  | In the above code, the maximum local vector size is set to one million 
  | elements. During  compilation of  an expression  if there  is a vector 
  | definition where the vector's size exceeds the maximum allowed  vector 
  | size  a  compilation  error shall  be  emitted.  The error  diagnostic 
  | generated by  the parser  on compilation  failure will  look like  the 
  | following: 
  |  
  |    ERR160 - Invalid vector size. Must be an integer in the 
  |    range [0,1000000], size: 1000000000 
  |  
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 12 - EXPRESSION STRUCTURES] 
  | Exprtk supports mathematical expressions in numerous forms based on  a 
  | simple  imperative  programming  model. This  section  will  cover the 
  | following  topics  related  to general  structure  and  programming of 
  | expressions using ExprTk: 
  |  
  |    (1) Multi-Statement Expressions 
  |    (2) Statements And Side-Effects 
  |    (3) Conditional Statements 
  |    (4) Special Functions 
  |  
  |  
  | (1) Multi-Statement Expressions 
  | Expressions in ExprTk can be comprised of one or more statements, which 
  | may  sometimes  be  called  sub-expressions.  The  following  are  two 
  | examples of expressions stored  in std::string variables, the  first a 
  | single statement and the second a multi-statement expression: 
  |  
  |    std::string single_statement = " z := x + y " 
  |  
  |    std::string multi_statement  = " var temp := x; " 
  |                                   " x := y + z;    " 
  |                                   " y := temp;     " 
  |  
  |  
  | In a  multi-statement expression,  the final  statement will determine 
  | the overall result of the expression. In the following multi-statement 
  | expression, the result of the expression when evaluated will be '2.3', 
  | which will also be the value stored in the 'y' variable. 
  |  
  |    z := x + y; 
  |    y := 2.3; 
  |  
  |  
  | As  demonstrated  in  the  expression  above,  statements  within   an 
  | expression are  separated using  the semi-colon  ';' operator.  In the 
  | event  two  statements are  not  separated by  a  semi-colon, and  the 
  | implied multiplication  feature is  active (enabled  by default),  the 
  | compiler  will  assume  a  multiplication  operation  between  the two 
  | statements. 
  |  
  | In the following example we have a multi-statement expression composed 
  | of two variable definitions and initialisations for variables x and  y 
  | and two seemingly separate mathematical operations. 
  |  
  |    var x:= 2; 
  |    var y:= 3; 
  |    x + 1 
  |    y * 2 
  |  
  |  
  | However the result of  the expression will not  be 6 as may  have been 
  | assumed based on  the calculation of  'y * 2',  but rather the  result 
  | will be 8. This  is because the compiler  will have conjoined the  two 
  | mathematical statements into one  via a multiplication operation.  The 
  | expression when compiled will actually evaluate as the following: 
  |  
  |    var x:= 2; 
  |    var y:= 3; 
  |    x + 1 * y * 2;   // 2 + 1 * 3 * 2 == 8 
  |  
  |  
  | In ExprTk any valid statement  will itself return a value.  This value 
  | can  further  be  used  in  conjunction  with  other  statements. This 
  | includes language structures such as if-statements, loops (for, while) 
  | and the switch statement. Typically the last statement executed in the 
  | given construct  (conditional, loop  etc), will  be the  value that is 
  | returned. 
  |  
  | In the following example, the  return value of the expression  will be 
  | 11, which is the sum of the variable 'x' and the final value  computed 
  | within the loop body upon its last iteration: 
  |  
  |    var x := 1; 
  |    x + for (var i := x; i < 10; i += 1) 
  |        { 
  |           i / 2; 
  |           i + 1; 
  |        } 
  |  
  |  
  | (2) Statements And Side-Effects 
  | Statements themselves may have side effects, which in-turn affects the 
  | proceeding statements in multi-statement expressions. 
  |  
  | A statement is said  to have a side-effect  if it causes the  state of 
  | the  expression to  change in  some way  -  this  includes but  is not 
  | limited to the  modification of the  state of external  variables used 
  | within the expression. Currently  the following actions being  present 
  | in a statement will cause it to have a side-effect: 
  |  
  |    (a) Assignment operation (explicit or potentially) 
  |    (b) Invoking a user-defined function that has side-effects 
  |  
  | The following are examples of expressions where the side-effect status 
  | of the  statements (sub-expressions) within the expressions  have been 
  | noted: 
  |  
  |   +-+----------------------+------------------------------+ 
  |   |#|      Expression      |      Side Effect Status      | 
  |   +-+----------------------+------------------------------+ 
  |   |0| x + y                | False                        | 
  |   +-+----------------------+------------------------------+ 
  |   |1| z := x + y           | True - Due to assignment     | 
  |   +-+----------------------+------------------------------+ 
  |   |2| abs(x - y)           | False                        | 
  |   +-+----------------------+------------------------------+ 
  |   |3| abs(x - y);          | False                        | 
  |   | | z := (x += y);       | True - Due to assignments    | 
  |   +-+----------------------+------------------------------+ 
  |   |4| var t := abs(x - y); | True - Due to initialisation | 
  |   | | t + x;               | False                        | 
  |   | | z := (x += y);       | True - Due to assignments    | 
  |   +-+----------------------+------------------------------+ 
  |   |5| foo(x - y)           | True - user defined function | 
  |   +-+----------------------+------------------------------+ 
  |  
  |  
  | Note09: In example  5 from  the above  set, it  is  assumed  the  user 
  | defined function foo has been  registered as having a side-effect.  By 
  | default all user defined  functions are assumed to  have side-effects, 
  | unless they  are configured  in their  constructors to  not have side- 
  | effects using the  'disable_has_side_effects' free function.  For more 
  | information review Section 15  - User Defined Functions  sub-section 7 
  | Function Side-Effects. 
  |  
  | At this point we  can see that there  will be expressions composed  of 
  | certain kinds  of statements  that when  executed will  not affect the 
  | nature  of  the  expression's result.  These statements  are typically 
  | called 'dead code'. These statements though not influencing the  final 
  | result will still be executed and as such they will consume processing 
  | time that could  otherwise be saved.  Consequently ExprTk attempts  to 
  | detect and remove such statements from expressions. 
  |  
  | The  'Dead  Code  Elimination' (DCE)  optimisation  process,  which is 
  | enabled by default, will remove any statements that are determined  to 
  | not have a side-effect in a multi-statement expression, excluding  the 
  | final or last statement. 
  |  
  | By default the final statement in an expression will always be present 
  | regardless of  its side-effect  status, as  it is  the statement whose 
  | value shall be used as the result of the expression. 
  |  
  | In order to further explain the actions taken during the DCE  process, 
  | lets review the following expression: 
  |  
  |    var x := 2;      // Statement 1 
  |    var y := x + 2;  // Statement 2 
  |    x + y;           // Statement 3 
  |    y := x + 3y;     // Statement 4 
  |    x - y;           // Statement 5 
  |  
  |  
  | The above expression has five statements.  Three of them (1, 2 and  4) 
  | actively have side-effects. The first two are variable declaration and 
  | initialisations, where as the third is due to an assignment operation. 
  | There  are  two  statements (3  and 5),  that do  not explicitly  have 
  | side-effects, however the latter, statement 5, is the final  statement 
  | in the expression and hence will be assumed to have a side-effect. 
  |  
  | During compilation when the DCE  optimisation is applied to the  above 
  | expression, statement 3 will be removed from the expression, as it has 
  | no  bearing  on  the  final result  of  expression,  the  rest of  the 
  | statements will all remain. The optimised form of the expression is as 
  | follows: 
  |  
  |    var x := 2;      // Statement 1 
  |    var y := x + 2;  // Statement 2 
  |    y := x + 3y;     // Statement 3 
  |    x - y;           // Statement 4 
  |  
  |  
  | (3) Conditional Statements (If-Then-Else) 
  | ExprTk supports two forms  of conditional branching or otherwise known 
  | as  if-statements.  The  first  form,  is  a  simple  function   based 
  | conditional  statement, that  takes exactly  three input  expressions: 
  | condition, consequent  and alternative.  The following  is an  example 
  | expression that utilises the function based if-statement. 
  |  
  |    x := if (y < z, y + 1, 2 * z) 
  |  
  |  
  | In the  example above,  if the  condition 'y  < z'  is true,  then the 
  | consequent 'y + 1' will be evaluated, its value shall be  returned and 
  | subsequently assigned to the  variable 'x'. Otherwise the  alternative 
  | '2 *  z' will  be evaluated  and its  value will  be returned. This is 
  | essentially the  simplest form of  an if-then-else statement. A simple 
  | variation of  the expression  where the  value of  the if-statement is 
  | used within another statement is as follows: 
  |  
  |    x := 3 * if (y < z, y + 1, 2 * z) / 2 
  |  
  |  
  | The second form of if-statement resembles the standard syntax found in 
  | most imperative languages. There are two variations of the statement: 
  |  
  |    (a) If-Statement 
  |    (b) If-Then-Else Statement 
  |  
  |  
  | (a) If-Statement 
  | This version  of the  conditional statement  returns the  value of the 
  | consequent expression when the  condition expression is true,  else it 
  | will return a quiet NaN value as its result. 
  |  
  |    Example 1: 
  |    x := if (y < z) y + 3; 
  |  
  |    Example 2: 
  |    x := if (y < z) 
  |         { 
  |            y + 3 
  |         }; 
  |  
  | The two  example expressions  above are  equivalent. If  the condition 
  | 'y < z' is true, the 'x' variable shall be  assigned the value of  the 
  | consequent 'y + 3', otherwise it  will be assigned the value of  quiet 
  | NaN.  As  previously  discussed,  if-statements  are  value  returning 
  | constructs, and if  not properly terminated  using a semi-colon,  will 
  | end-up  combining  with  the  next  statement  via  a   multiplication 
  | operation. The following example will NOT result in the expected value 
  | of 'w + x' being returned: 
  |  
  |    x := if (y < z) y + 3  // missing semi-colon ';' 
  |    w + x 
  |  
  |  
  | When the  above supposed  multi-statement expression  is compiled, the 
  | expression  will  have  a  multiplication  inserted  between  the  two 
  | 'intended' statements resulting in the unanticipated expression: 
  |  
  |    x := (if (y < z) y + 3) * w + x 
  |  
  |  
  | The  solution  to  the  above situation  is  to  simply  terminate the 
  | conditional statement with a semi-colon as follows: 
  |  
  |    x := if (y < z) y + 3; 
  |    w + x 
  |  
  |  
  | (b) If-Then-Else Statement 
  | The second variation of  the if-statement is to  allow for the use  of 
  | Else and Else-If cascading statements. Examples of such statements are 
  | as follows: 
  |  
  |  Example 1:             Example 2:         Example 3: 
  |  if (x < y)             if (x < y)         if (x > y + 1) 
  |     z := x + 3;         {                     y := abs(x - z); 
  |  else                      y := z + x;     else 
  |     y := x - z;            z := x + 3;     { 
  |                         }                     y := z + x; 
  |                         else                  z := x + 3; 
  |                            y := x - z;     }; 
  |  
  |  
  |  Example 4:             Example 5:         Example 6: 
  |  if (2 * x < max(y,3))  if (x < y)         if (x < y or (x + z) > y) 
  |  {                         z := x + 3;     { 
  |     y := z + x;         else if (2y != z)     z := x + 3; 
  |     z := x + 3;         {                     y := x - z; 
  |  }                         z := x + 3;     } 
  |  else if (2y - z)          y := x - z;     else if (abs(2y - z) >= 3) 
  |     y := x - z;         }                     y := x - z; 
  |                         else               else 
  |                            x * x;          { 
  |                                               z := abs(x * x); 
  |                                               x * y * z; 
  |                                            }; 
  |  
  |  
  | In  the case  where  there  is no  final else  statement and  the flow 
  | through the conditional  arrives at this  final point, the  same rules 
  | apply to this form of if-statement as to the previous. That is a quiet 
  | NaN shall be returned as the  result of the  if-statement. Furthermore 
  | the same requirements of  terminating the statement with  a semi-colon 
  | apply. 
  |  
  | (4) Special Functions 
  | The purpose  of special  functions in  ExprTk is  to provide  compiler 
  | generated equivalents of common mathematical expressions which can  be 
  | invoked by  using the  'special function'  syntax (eg:  $f12(x,y,z) or 
  | $f82(x,y,z,w)). 
  |  
  | Special functions dramatically decrease  the total evaluation time  of 
  | expressions which would otherwise  have been written using  the common 
  | form by reducing the total number  of nodes in the evaluation tree  of 
  | an  expression  and  by  also  leveraging  the  compiler's  ability to 
  | correctly optimise such expressions for a given architecture. 
  |  
  |           3-Parameter                       4-Parameter 
  |  +-------------+-------------+    +--------------+------------------+ 
  |  |  Prototype  |  Operation  |    |  Prototype   |    Operation     | 
  |  +-------------+-------------+    +--------------+------------------+ 
  |    $f00(x,y,z) | (x + y) / z       $f48(x,y,z,w) | x + ((y + z) / w) 
  |    $f01(x,y,z) | (x + y) * z       $f49(x,y,z,w) | x + ((y + z) * w) 
  |    $f02(x,y,z) | (x + y) - z       $f50(x,y,z,w) | x + ((y - z) / w) 
  |    $f03(x,y,z) | (x + y) + z       $f51(x,y,z,w) | x + ((y - z) * w) 
  |    $f04(x,y,z) | (x - y) + z       $f52(x,y,z,w) | x + ((y * z) / w) 
  |    $f05(x,y,z) | (x - y) / z       $f53(x,y,z,w) | x + ((y * z) * w) 
  |    $f06(x,y,z) | (x - y) * z       $f54(x,y,z,w) | x + ((y / z) + w) 
  |    $f07(x,y,z) | (x * y) + z       $f55(x,y,z,w) | x + ((y / z) / w) 
  |    $f08(x,y,z) | (x * y) - z       $f56(x,y,z,w) | x + ((y / z) * w) 
  |    $f09(x,y,z) | (x * y) / z       $f57(x,y,z,w) | x - ((y + z) / w) 
  |    $f10(x,y,z) | (x * y) * z       $f58(x,y,z,w) | x - ((y + z) * w) 
  |    $f11(x,y,z) | (x / y) + z       $f59(x,y,z,w) | x - ((y - z) / w) 
  |    $f12(x,y,z) | (x / y) - z       $f60(x,y,z,w) | x - ((y - z) * w) 
  |    $f13(x,y,z) | (x / y) / z       $f61(x,y,z,w) | x - ((y * z) / w) 
  |    $f14(x,y,z) | (x / y) * z       $f62(x,y,z,w) | x - ((y * z) * w) 
  |    $f15(x,y,z) | x / (y + z)       $f63(x,y,z,w) | x - ((y / z) / w) 
  |    $f16(x,y,z) | x / (y - z)       $f64(x,y,z,w) | x - ((y / z) * w) 
  |    $f17(x,y,z) | x / (y * z)       $f65(x,y,z,w) | ((x + y) * z) - w 
  |    $f18(x,y,z) | x / (y / z)       $f66(x,y,z,w) | ((x - y) * z) - w 
  |    $f19(x,y,z) | x * (y + z)       $f67(x,y,z,w) | ((x * y) * z) - w 
  |    $f20(x,y,z) | x * (y - z)       $f68(x,y,z,w) | ((x / y) * z) - w 
  |    $f21(x,y,z) | x * (y * z)       $f69(x,y,z,w) | ((x + y) / z) - w 
  |    $f22(x,y,z) | x * (y / z)       $f70(x,y,z,w) | ((x - y) / z) - w 
  |    $f23(x,y,z) | x - (y + z)       $f71(x,y,z,w) | ((x * y) / z) - w 
  |    $f24(x,y,z) | x - (y - z)       $f72(x,y,z,w) | ((x / y) / z) - w 
  |    $f25(x,y,z) | x - (y / z)       $f73(x,y,z,w) | (x * y) + (z * w) 
  |    $f26(x,y,z) | x - (y * z)       $f74(x,y,z,w) | (x * y) - (z * w) 
  |    $f27(x,y,z) | x + (y * z)       $f75(x,y,z,w) | (x * y) + (z / w) 
  |    $f28(x,y,z) | x + (y / z)       $f76(x,y,z,w) | (x * y) - (z / w) 
  |    $f29(x,y,z) | x + (y + z)       $f77(x,y,z,w) | (x / y) + (z / w) 
  |    $f30(x,y,z) | x + (y - z)       $f78(x,y,z,w) | (x / y) - (z / w) 
  |    $f31(x,y,z) | x * y^2 + z       $f79(x,y,z,w) | (x / y) - (z * w) 
  |    $f32(x,y,z) | x * y^3 + z       $f80(x,y,z,w) | x / (y + (z * w)) 
  |    $f33(x,y,z) | x * y^4 + z       $f81(x,y,z,w) | x / (y - (z * w)) 
  |    $f34(x,y,z) | x * y^5 + z       $f82(x,y,z,w) | x * (y + (z * w)) 
  |    $f35(x,y,z) | x * y^6 + z       $f83(x,y,z,w) | x * (y - (z * w)) 
  |    $f36(x,y,z) | x * y^7 + z       $f84(x,y,z,w) | x*y^2 + z*w^2 
  |    $f37(x,y,z) | x * y^8 + z       $f85(x,y,z,w) | x*y^3 + z*w^3 
  |    $f38(x,y,z) | x * y^9 + z       $f86(x,y,z,w) | x*y^4 + z*w^4 
  |    $f39(x,y,z) | x * log(y)+z      $f87(x,y,z,w) | x*y^5 + z*w^5 
  |    $f40(x,y,z) | x * log(y)-z      $f88(x,y,z,w) | x*y^6 + z*w^6 
  |    $f41(x,y,z) | x * log10(y)+z    $f89(x,y,z,w) | x*y^7 + z*w^7 
  |    $f42(x,y,z) | x * log10(y)-z    $f90(x,y,z,w) | x*y^8 + z*w^8 
  |    $f43(x,y,z) | x * sin(y)+z      $f91(x,y,z,w) | x*y^9 + z*w^9 
  |    $f44(x,y,z) | x * sin(y)-z      $f92(x,y,z,w) | (x and y) ? z : w 
  |    $f45(x,y,z) | x * cos(y)+z      $f93(x,y,z,w) | (x or  y) ? z : w 
  |    $f46(x,y,z) | x * cos(y)-z      $f94(x,y,z,w) | (x <   y) ? z : w 
  |    $f47(x,y,z) | x ? y : z         $f95(x,y,z,w) | (x <=  y) ? z : w 
  |                                    $f96(x,y,z,w) | (x >   y) ? z : w 
  |                                    $f97(x,y,z,w) | (x >=  y) ? z : w 
  |                                    $f98(x,y,z,w) | (x ==  y) ? z : w 
  |                                    $f99(x,y,z,w) | x*sin(y)+z*cos(w) 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 13 - VARIABLE, VECTOR & STRING DEFINITION] 
  | ExprTk supports the definition of expression local variables,  vectors 
  | and  strings.  The definitions  must  be unique  as  shadowing is  not 
  | allowed and object lifetimes are based  on scope. Definitions use  the 
  | following general form: 
  |  
  |    var <name> := <initialiser>; 
  |  
  | (1) Variable Definition 
  | Variables are  of numeric  type denoting  a single  value. They can be 
  | explicitly initialised to a value, otherwise they will be defaulted to 
  | zero. The following are examples of variable definitions: 
  |  
  |    (a) Initialise x to zero 
  |        var x; 
  |  
  |    (b) Initialise y to three 
  |        var y := 3; 
  |  
  |    (c) Initialise z to the expression 
  |        var z := if (max(1, x + y) > 2, w, v); 
  |  
  |    (d) Initialise const literal n 
  |        var n := 12 / 3; 
  |  
  |  
  | (2) Vector Definition 
  | Vectors are arrays of a common numeric type. The elements in a  vector 
  | can be explicitly initialised, otherwise they will all be defaulted to 
  | zero. The following are examples of vector definitions: 
  |  
  |    (a) Initialise all values to zero 
  |        var x[3]; 
  |  
  |    (b) Initialise all values to zero 
  |        var x[3] := {}; 
  |  
  |    (c) Initialise all values to given value or expression 
  |        var x[3]   := [ 42 ]; 
  |        var y[x[]] := [ 123 + 3y + sin(w / z) ]; 
  |  
  |    (d) Initialise all values iota style 
  |        var v[4] := [ 0 : +1];  //  0,  1,  2,  3 
  |        var v[5] := [-3 : -2];  // -3, -5, -7, -9, -11 
  |  
  |    (e) Initialise the first two values, all other elements to zero 
  |        var x[3] := { (1 + x[2]) / x[], (sin(y[0] / x[]) + 3) / x[] }; 
  |  
  |    (f) Initialise the first three (all) values 
  |        const var size := 3; 
  |        var x[size] := { 1, 2, 3 }; 
  |  
  |    (g) Initialise vector from a vector 
  |        var x[4] := { 1, 2, 3, 4 }; 
  |        var y[3] := x; 
  |        var w[5] := { 1, 2 }; // 1, 2, 0, 0, 0 
  |  
  |    (h) Initialise vector from a smaller vector 
  |        var x[3] := { 1, 2, 3 }; 
  |        var y[5] := x;   // 1, 2, 3, ??, ?? 
  |  
  |    (i) Non-initialised vector 
  |        var x[3] := null; // ?? ?? ?? 
  |  
  |    (j) Error as there are too many initialisers 
  |        var x[3] := { 1, 2, 3, 4 }; 
  |  
  |    (k) Error as a vector of size zero is not allowed. 
  |        var x[0]; 
  |  
  |  
  | (3) String Definition 
  | Strings are sequences comprised of 8-bit characters. They can only be 
  | defined  with an explicit  initialisation  value. The  following  are 
  | examples of string variable definitions: 
  |  
  |    (a) Initialise to a string 
  |        var x := 'abc'; 
  |  
  |    (b) Initialise to an empty string 
  |        var x := ''; 
  |  
  |    (c) Initialise to a string expression 
  |        var x := 'abc' + '123'; 
  |  
  |    (d) Initialise to a string range 
  |        var x := 'abc123'[2:4]; 
  |  
  |    (e) Initialise to another string variable 
  |        var x := 'abc'; 
  |        var y := x; 
  |  
  |    (f) Initialise to another string variable range 
  |        var x := 'abc123'; 
  |        var y := x[2:4]; 
  |  
  |    (g) Initialise to a string expression 
  |        var x := 'abc'; 
  |        var y := x + '123'; 
  |  
  |    (h) Initialise to a string expression range 
  |        var x := 'abc'; 
  |        var y := (x + '123')[1:3]; 
  |  
  |  
  | (4) Return Value 
  | Variable and vector  definitions have a  return value. In  the case of 
  | variable definitions, the value  to which the variable  is initialised 
  | will be returned. Where as for vectors, the value of the first element 
  | (eg: v[0]) shall be returned. 
  |  
  |    8 == ((var x := 7;) + 1) 
  |    4 == (var y[3] := {4, 5, 6};) 
  |  
  |  
  | (5) Variable/Vector Assignment 
  | The value of a variable can be assigned to a vector and a vector or  a 
  | vector expression can be assigned to a variable. 
  |  
  |   (a) Variable To Vector: 
  |       Every element of the vector is assigned the value of the variable 
  |       or expression. 
  |       var x    := 3; 
  |       var y[3] := { 1, 2, 3 }; 
  |       y := x + 1; 
  |  
  |   (b) Vector To Variable: 
  |       The variable is assigned the value of the first element of the 
  |       vector (aka vec[0]) 
  |       var x    := 3; 
  |       var y[3] := { 1, 2, 3 }; 
  |       x := y + 1; 
  |  
  |  
  | Note10: During the expression compilation phase, tokens are classified 
  | based on the following priorities: 
  |  
  |    (a) Reserved keywords or operators (+, -, and, or, etc) 
  |    (b) Base functions (abs, sin, cos, min, max etc) 
  |    (c) Symbol table variables 
  |    (d) Expression local defined variables 
  |    (e) Symbol table functions 
  |    (f) Unknown symbol resolver based variables 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 14 - VECTOR PROCESSING] 
  | ExprTk  provides  support  for   various  forms  of  vector   oriented 
  | arithmetic, inequalities and  processing. The various  supported pairs 
  | are as follows: 
  |  
  |    (a) vector and vector (eg: v0 + v1) 
  |    (b) vector and scalar (eg: v  + 33) 
  |    (c) scalar and vector (eg: 22 *  v) 
  |  
  | The following is a list of operations that can be used in  conjunction 
  | with vectors: 
  |  
  |    (a) Arithmetic:       +, -, *, /, % 
  |    (b) Exponentiation:   vector ^ scalar 
  |    (c) Assignment:       :=, +=, -=, *=, /=, %=, <=> 
  |    (d) Inequalities:     <, <=, >, >=, ==, =, equal 
  |    (e) Boolean logic:    and, nand, nor, or, xnor, xor 
  |    (f) Unary operations: 
  |        abs, acos, acosh, asin, asinh, atan, atanh, ceil, cos,  cosh, 
  |        cot, csc,  deg2grad, deg2rad,  erf, erfc,  exp, expm1, floor, 
  |        frac, grad2deg, log, log10, log1p, log2, rad2deg, round, sec, 
  |        sgn, sin, sinc, sinh, sqrt, swap, tan, tanh, trunc, 
  |        thresholding 
  |    (g) Aggregate and Reduce operations: 
  |        avg, max, min, mul,  dot, dotk, sum, sumk,  count, all_true, 
  |        all_false, any_true, any_false 
  |    (h) Transformation operations: 
  |        copy, diff, reverse, rotate-left/right, shift-left/right, sort, 
  |        nth_element 
  |    (i) BLAS-L1: 
  |        axpy, axpby, axpyz, axpbyz, axpbz 
  |  
  | Note11: When one of the above  described operations is being performed 
  | between two  vectors, the  operation will  only span  the size  of the 
  | smallest vector.  The elements  of the  larger vector  outside of  the 
  | range will  not be included. The  operation  itself will  be processed 
  | element-wise over values of the smaller of the two ranges. 
  |  
  | The  following  simple  example  demonstrates  the  vector  processing 
  | capabilities by computing the dot-product of the vectors v0 and v1 and 
  | then assigning it to the variable v0dotv1: 
  |  
  |    var v0[3] := { 1, 2, 3 }; 
  |    var v1[3] := { 4, 5, 6 }; 
  |    var v0dotv1 := sum(v0 * v1); 
  |  
  |  
  | The following is a for-loop based implementation that is equivalent to 
  | the previously mentioned dot-product computation expression: 
  |  
  |    var v0[3] := { 1, 2, 3 }; 
  |    var v1[3] := { 4, 5, 6 }; 
  |    var v0dotv1; 
  |  
  |    for (var i := 0; i < min(v0[],v1[]); i += 1) 
  |    { 
  |       v0dotv1 += (v0[i] * v1[i]); 
  |    } 
  |  
  |  
  | Note12: When the aggregate or reduction  operations denoted above  are 
  | used  in conjunction with a  vector or  vector  expression, the return 
  | value is not a vector but rather a single value. 
  |  
  |    var x[3] := { 1, 2, 3 }; 
  |  
  |    sum(x)      ==  6 
  |    sum(1 + 2x) == 15 
  |    avg(3x + 1) ==  7 
  |    min(1 / x)  == (1 / 3) 
  |    max(x / 2)  == (3 / 2) 
  |    sum(x > 0 and x < 5) == x[] 
  |  
  |  
  | When utilising external user defined  vectors via the symbol table  as 
  | opposed to expression local defined vectors, the typical  'add_vector' 
  | method from the symbol table will register the entirety of the  vector 
  | that is passed. The following example attempts to evaluate the sum  of 
  | elements of  the external  user defined  vector within  a typical  yet 
  | trivial expression: 
  |  
  |    const std::string reduce_program = " sum(2 * v + 1) " 
  |  
  |    std::vector<T> v0 { T(1.1), T(2.2), ..... , T(99.99) }; 
  |  
  |    symbol_table_t symbol_table; 
  |    symbol_table.add_vector("v",v); 
  |  
  |    expression_t expression; 
  |    expression.register_symbol_table(symbol_table); 
  |  
  |    parser_t parser; 
  |    parser.compile(reduce_program,expression); 
  |  
  |    T sum = expression.value(); 
  |  
  |  
  | For the most part, this is  a very common use-case. However there  may 
  | be situations where one may want to evaluate the same vector  oriented 
  | expression many times over, but using different vectors or sub  ranges 
  | of the same vector of the same size to that of the original upon every 
  | evaluation. 
  |  
  | The usual solution is to  either recompile the expression for  the new 
  | vector instance, or to  copy the contents from  the new vector to  the 
  | symbol table registered vector  and then perform the  evaluation. When 
  | the  vectors are  large or  the re-evaluation  attempts are  numerous, 
  | these  solutions  can  become  rather  time  consuming  and  generally 
  | inefficient. 
  |  
  |    std::vector<T> v1 { T(2.2), T(2.2), ..... , T(2.2) }; 
  |    std::vector<T> v2 { T(3.3), T(3.3), ..... , T(3.3) }; 
  |    std::vector<T> v3 { T(4.4), T(4.4), ..... , T(4.4) }; 
  |  
  |    std::vector<std::vector<T>> vv { v1, v2, v3 }; 
  |    ... 
  |    T sum = T(0); 
  |  
  |    for (auto& new_vec : vv) 
  |    { 
  |       v = new_vec; // update vector 
  |       sum += expression.value(); 
  |    } 
  |  
  |  
  | A  solution  to  the  above  'efficiency'  problem,  is  to  use   the 
  | exprtk::vector_view  object. The  vector_view is  instantiated with  a 
  | size and backing based upon a vector. Upon evaluations if the  backing 
  | needs  to  be  'updated' to  either another  vector or  sub-range, the 
  | vector_view instance  can be  efficiently rebased,  and the expression 
  | evaluated as normal. 
  |  
  |    exprtk::vector_view<T> view = exprtk::make_vector_view(v,v.size()); 
  |  
  |    symbol_table_t symbol_table; 
  |    symbol_table.add_vector("v",view); 
  |  
  |    ... 
  |  
  |    T sum = T(0); 
  |  
  |    for (auto& new_vec : vv) 
  |    { 
  |       view.rebase(new_vec.data()); // update vector 
  |       sum += expression.value(); 
  |    } 
  |  
  |  
  | Another useful feature of exprtk::vector_view is that all such vectors 
  | can have  their sizes  modified (or  "resized"). The  resizing of  the 
  | associated vectors can happen either between or during evaluations. 
  |  
  |    std::vector<T> v = { 1, 2, 3, 4, 5, 6, 7, 8 }; 
  |    exprtk::vector_view<T> view = exprtk::make_vector_view(v,v.size()); 
  |  
  |    symbol_table_t symbol_table; 
  |    symbol_table.add_vector("v",view); 
  |  
  |    const std::string expression_string = "v[]" 
  |  
  |    expression_t expression; 
  |    expression.register_symbol_table(symbol_table); 
  |  
  |    parser_t parser; 
  |    parser.compile(expression_string, expression); 
  |  
  |    for (std::size_t i = 1; i <= v.size(); ++i) 
  |    { 
  |       vv.set_size(i); 
  |       expression.value(); 
  |    } 
  |  
  |  
  | In the example above, a vector_view is instantiated with a std::vector 
  | instance with eight elements and registered to the given symbol_table. 
  | An expression is then compiled,  which in this case simply returns the 
  | size of the vector at that point in time. The expression is  evaluated 
  | eight times (size of vector times), where upon each iteration the size 
  | of the vector is changed with values ranging from one to eight. 
  |  
  | Note13: When modifying the size of  a vector, the  new size must be at 
  | least one  or larger  and must  not exceed  the original  size of  the 
  | vector_view when it was instantiated. 
  |  
  | Note14: The  lifetime  of  any  parser,  symbol_table   or  expression 
  | instance must  not exceed  that of  any vector_view  instance that has 
  | been registered  with it.  Furthermore the  lifetime of  a vector_view 
  | must not exceed that of the underlying vector instance it is 
  | associated with. 
  |  
  | Note15: In a multi-threaded context the  rebase function should not be 
  | called during associated expression  evaluation, as this will  lead to 
  | undefined behaviour (eg: torn reads and writes). 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 15 - USER DEFINED FUNCTIONS] 
  | ExprTk provides a means  whereby custom functions can  be defined  and 
  | utilised within  expressions.  The   concept  requires  the  user   to 
  | provide a reference  to the function  coupled with an  associated name 
  | that  will be invoked within  expressions. Functions may take numerous 
  | inputs but will always return a single value of the underlying numeric 
  | type. 
  |  
  | During  expression  compilation  when required  the  reference  to the 
  | function  shall be obtained from  the associated  symbol_table and  be 
  | embedded into the expression. 
  |  
  | There are six types of function interface: 
  |  
  |   +---+----------------------+--------------+----------------------+ 
  |   | # |         Name         | Return Type  | Input Types          | 
  |   +---+----------------------+--------------+----------------------+ 
  |   | 1 | ifunction            | Scalar       | Scalar               | 
  |   | 2 | ivararg_function     | Scalar       | Scalar               | 
  |   | 3 | igeneric_function    | Scalar       | Scalar,Vector,String | 
  |   | 4 | igeneric_function II | String       | Scalar,Vector,String | 
  |   | 5 | igeneric_function III| String/Scalar| Scalar,Vector,String | 
  |   | 6 | function_compositor  | Scalar       | Scalar               | 
  |   +---+----------------------+--------------+----------------------+ 
  |  
  | (1) ifunction 
  | This interface supports zero to 20 input parameters of only the scalar 
  | type (numbers). The  usage requires a custom function be  derived from 
  | ifunction and to override one of the 21 function operators. As part of 
  | the constructor the custom function will define how many parameters it 
  | expects  to  handle.  The  following  example  defines  a  3 parameter 
  | function called 'foo': 
  |  
  |    template <typename T> 
  |    struct foo final : public exprtk::ifunction<T> 
  |    { 
  |       foo() : exprtk::ifunction<T>(3) 
  |       {} 
  |  
  |       T operator()(const T& v1, const T& v2, const T& v3) override 
  |       { 
  |          return T(1) + (v1 * v2) / T(v3); 
  |       } 
  |    }; 
  |  
  |  
  | (2) ivararg_function 
  | This interface supports a variable number of scalar arguments as input 
  | into the function. The function operator interface uses a  std::vector 
  | specialised upon type T to facilitate parameter passing. The following 
  | example defines a vararg function called 'boo': 
  |  
  |    template <typename T> 
  |    struct boo final : public exprtk::ivararg_function<T> 
  |    { 
  |       inline T operator()(const std::vector<T>& arglist) override 
  |       { 
  |          T result = T(0); 
  |  
  |          for (std::size_t i = 0; i < arglist.size(); ++i) 
  |          { 
  |             result += arglist[i] / arglist[i > 0 ? (i - 1) : 0]; 
  |          } 
  |  
  |          return result; 
  |       } 
  |    }; 
  |  
  |  
  | (3) igeneric_function 
  | This interface supports  a variable number  of arguments and  types as 
  | input  into  the  function. The  function  operator  interface uses  a 
  | std::vector  specialised  upon  the  type_store  type  to   facilitate 
  | parameter passing. 
  |  
  |     Scalar <-- function(i_0, i_1, i_2....., i_N) 
  |  
  |  
  | The  fundamental  types  that  can  be  passed  into  the  function as 
  | parameters and their views are as follows: 
  |  
  |    (1) Scalar - scalar_view 
  |    (2) Vector - vector_view 
  |    (3) String - string_view 
  |  
  |  
  | The above denoted type  views provide non-const reference-like  access 
  | to each parameter, as such modifications made to the input  parameters 
  | will  persist after  the function  call has  completed. The  following 
  | example defines a generic function called 'too': 
  |  
  |    template <typename T> 
  |    struct too final : public exprtk::igeneric_function<T> 
  |    { 
  |       typedef typename exprtk::igeneric_function<T>::parameter_list_t 
  |                                                      parameter_list_t; 
  |  
  |       too() 
  |       {} 
  |  
  |       inline T operator()(parameter_list_t parameters) override 
  |       { 
  |          for (std::size_t i = 0; i < parameters.size(); ++i) 
  |          { 
  |             ... 
  |          } 
  |  
  |          return T(0); 
  |       } 
  |    }; 
  |  
  |  
  | In the example above, the input 'parameters' to the function operator, 
  | parameter_list_t,  is  a  type  of  std::vector  of  type_store.  Each 
  | type_store  instance  has  a  member  called  'type'  which  holds the 
  | enumeration pertaining to the underlying type of the type_store. There 
  | are three type enumerations: 
  |  
  |    (1) e_scalar - literals, variables, vector elements, expressions 
  |        eg: 123.456, x, vec[3x + 1], 2x + 3 
  |  
  |    (2) e_vector - vectors, vector expressions 
  |        eg: vec1, 2 * vec1 + vec2 / 3 
  |  
  |    (3) e_string - strings, string literals and range variants of both 
  |        eg: 'AString', s0, 'AString'[x:y], s1[1 + x:] + 'AString' 
  |  
  |  
  | Each of the  parameters can be  accessed using its  designated view. 
  | A typical loop for processing the parameters is as follows: 
  |  
  |    inline T operator()(parameter_list_t parameters) 
  |    { 
  |       typedef typename exprtk::igeneric_function<T>::generic_type 
  |                                                      generic_type; 
  |  
  |       typedef typename generic_type::scalar_view scalar_t; 
  |       typedef typename generic_type::vector_view vector_t; 
  |       typedef typename generic_type::string_view string_t; 
  |  
  |       for (std::size_t i = 0; i < parameters.size(); ++i) 
  |       { 
  |          generic_type& gt = parameters[i]; 
  |  
  |          if (generic_type::e_scalar == gt.type) 
  |          { 
  |             scalar_t x(gt); 
  |             ... 
  |          } 
  |          else if (generic_type::e_vector == gt.type) 
  |          { 
  |             vector_t vector(gt); 
  |             ... 
  |          } 
  |          else if (generic_type::e_string == gt.type) 
  |          { 
  |             string_t string(gt); 
  |             ... 
  |          } 
  |       } 
  |  
  |       return T(0); 
  |    } 
  |  
  |  
  | Most often than not a custom generic function will require a  specific 
  | sequence of parameters, rather than some arbitrary sequence of  types. 
  | In those situations, ExprTk can perform compile-time type checking  to 
  | validate that function invocations  are carried out using  the correct 
  | sequence of parameters. Furthermore  performing the checks at  compile 
  | -time rather than at run-time (aka every time the function is invoked) 
  | will result in expression evaluation performance gains. 
  |  
  | Compile-time type  checking of  input parameters  can be  requested by 
  | passing  a string  to the  constructor of  the igeneric_function  that 
  | represents the required sequence of parameter types. When no parameter 
  | sequence is provided, it is implied the function can accept a variable 
  | number of parameters comprised of any of the fundamental types. 
  |  
  | Each fundamental type has an  associated character. The following is a 
  | listing of said characters and their meanings: 
  |  
  |    (1) T - Scalar 
  |    (2) V - Vector 
  |    (3) S - String 
  |    (4) Z - Zero or no parameters 
  |    (5) ? - Any type (Scalar, Vector or String) 
  |    (6) * - Wildcard operator 
  |    (7) | - Parameter sequence delimiter 
  |  
  |  
  | No other characters other than the seven denoted above may be included 
  | in the parameter sequence  definition. If any such  invalid characters 
  | do exist, registration of the associated generic function to a  symbol 
  | table ('add_function' method) will fail. If the parameter sequence  is 
  | modified resulting in it becoming  invalid after having been added  to 
  | the symbol table but before the compilation step, a compilation  error 
  | will be incurred. 
  |  
  | The  following   example  demonstrates   a  simple   generic  function 
  | implementation with a user specified parameter sequence: 
  |  
  |    template <typename T> 
  |    struct moo final : public exprtk::igeneric_function<T> 
  |    { 
  |       typedef typename exprtk::igeneric_function<T>::parameter_list_t 
  |                                                      parameter_list_t; 
  |  
  |       moo() 
  |       : exprtk::igeneric_function<T>("SVTT") 
  |       {} 
  |  
  |       inline T operator()(parameter_list_t parameters) override 
  |       { 
  |          ... 
  |       } 
  |    }; 
  |  
  |  
  | In the example above the  generic function 'moo' expects exactly  four 
  | parameters in the following sequence: 
  |  
  |    (1) String 
  |    (2) Vector 
  |    (3) Scalar 
  |    (4) Scalar 
  |  
  | Note16: The 'Z' or no parameter option may not be used in  conjunction 
  | with any other type option in a parameter sequence. When  incorporated 
  | in the parameter  sequence list, the  'No Parameter' option  indicates 
  | that the function may be invoked without any parameters being  passed. 
  | For more information refer to the section: 'Zero Parameter Functions' 
  |  
  |  
  | (4) igeneric_function II 
  | This interface is identical to  the igeneric_function, in that in  can 
  | consume an  arbitrary number  of parameters  of varying  type, but the 
  | difference being  that the  function returns  a string  and as such is 
  | treated as a string when  invoked within expressions. As a  result the 
  | function call can  alias a string  and interact with  other strings in 
  | situations such as concatenation and equality operations. 
  |  
  |     String <-- function(i_0, i_1, i_2....., i_N) 
  |  
  |  
  | The following example defines a generic function  named 'toupper' with 
  | the string return type function operator being explicitly overridden: 
  |  
  |    template <typename T> 
  |    struct toupper final : public exprtk::igeneric_function<T> 
  |    { 
  |       typedef exprtk::igeneric_function<T> igenfunct_t; 
  |       typedef typename igenfunct_t::generic_type generic_t; 
  |       typedef typename igenfunct_t::parameter_list_t parameter_list_t; 
  |       typedef typename generic_t::string_view string_t; 
  |  
  |       toupper() 
  |       : exprtk::igeneric_function<T>("S",igenfunct_t::e_rtrn_string) 
  |       {} 
  |  
  |       inline T operator()(std::string& result, 
  |                           parameter_list_t parameters) override 
  |       { 
  |          result.clear(); 
  |  
  |          string_t string(parameters[0]); 
  |  
  |          for (std::size_t i = 0; i < string.size(); ++i) 
  |          { 
  |             result += std::toupper(string[i]); 
  |          } 
  |  
  |          return T(0); 
  |       } 
  |    }; 
  |  
  |  
  | In the example above the  generic function 'toupper' expects only  one 
  | input parameter  of type  string, as  noted by  the parameter sequence 
  | string passed during the  constructor. Furthermore a second  parameter 
  | is passed to the constructor indicating that it should be treated as a 
  | string returning function -  by default it is  assumed to be a  scalar 
  | returning function. 
  |  
  | When executed,  the function  will return  as a  result a  copy of the 
  | input string converted to uppercase form. An example expression  using 
  | the toupper function registered as the symbol 'toupper' is as follows: 
  |  
  |    "'ABCDEF' == toupper('aBc') + toupper('DeF')" 
  |  
  |  
  | Note17: When  adding a  string type  returning generic  function to  a 
  | symbol  table  the  'add_function'  is  invoked.  The  example   below 
  | demonstrates how this can be done: 
  |  
  |    toupper<T> tu; 
  |  
  |    exprtk::symbol_table<T> symbol_table; 
  |  
  |    symbol_table.add_function("toupper",tu); 
  |  
  |  
  | Note18: Two further refinements to the  type checking facility are the 
  | possibilities  of  a variable  number  of common  types  which can  be 
  | accomplished by using a wildcard '*' and a special 'any type' which is 
  | done using  the '?'  character. It  should be  noted that the wildcard 
  | operator is  associated with  the previous  type in  the sequence  and 
  | implies one or more of that type. 
  |  
  |    template <typename T> 
  |    struct zoo final : public exprtk::igeneric_function<T> 
  |    { 
  |       typedef typename exprtk::igeneric_function<T>::parameter_list_t 
  |                                                      parameter_list_t; 
  |  
  |       zoo() 
  |       : exprtk::igeneric_function<T>("SVT*V?") 
  |       {} 
  |  
  |       inline T operator()(parameter_list_t parameters) override 
  |       { 
  |          ... 
  |       } 
  |    }; 
  |  
  |  
  | In the example above the generic function 'zoo' expects at least  five 
  | parameters in the following sequence: 
  |  
  |    (1) String 
  |    (2) Vector 
  |    (3) One or more Scalars 
  |    (4) Vector 
  |    (5) Any type (one type of either a scalar, vector or string) 
  |  
  |  
  | A final  piece of  type checking  functionality is  available for  the 
  | scenarios where  a single  function name  is intended  to be  used for 
  | multiple distinct parameter sequences,  another name for this  feature 
  | is function  overloading. The  parameter sequences  are passed  to the 
  | constructor as a  single string delimited  by the pipe  '|' character. 
  | Two specific overrides of the  function operator are provided one  for 
  | standard generic functions and one for string returning functions. The 
  | overrides are as follows: 
  |  
  |       // Scalar <-- function(psi,i_0,i_1,....,i_N) 
  |       inline T operator()(const std::size_t& ps_index, 
  |                           parameter_list_t parameters) 
  |       { 
  |          ... 
  |       } 
  |  
  |       // String <-- function(psi,i_0,i_1,....,i_N) 
  |       inline T operator()(const std::size_t& ps_index, 
  |                           std::string& result, 
  |                           parameter_list_t parameters) 
  |       { 
  |          ... 
  |       } 
  |  
  |  
  | When the function  operator is invoked  the 'ps_index' parameter  will 
  | have as its value the index of the parameter sequence that matches the 
  | specific invocation. This way complex and time consuming type checking 
  | conditions need not  be executed in  the function itself  but rather a 
  | simple and efficient  dispatch to a  specific implementation for  that 
  | particular parameter sequence can be performed. 
  |  
  |    template <typename T> 
  |    struct roo final : public exprtk::igeneric_function<T> 
  |    { 
  |       typedef typename exprtk::igeneric_function<T>::parameter_list_t 
  |                                                      parameter_list_t; 
  |  
  |       moo() 
  |       : exprtk::igeneric_function<T>("SVTT|SS|TTV|S?V*S") 
  |       {} 
  |  
  |       inline T operator()(const std::size_t& ps_index, 
  |                           parameter_list_t parameters) override 
  |       { 
  |          ... 
  |       } 
  |    }; 
  |  
  |  
  | In the example above there are four distinct parameter sequences  that 
  | can be processed  by the generic  function 'roo'. Any  other parameter 
  | sequences will cause a compilation error. The four valid sequences are 
  | as follows: 
  |  
  |     Sequence-0    Sequence-1    Sequence-2    Sequence-3 
  |       'SVTT'         'SS'          'TTV'       'S?V*S' 
  |    (1) String    (1) String    (1) Scalar    (1) String 
  |    (2) Vector    (2) String    (2) Scalar    (2) Any Type 
  |    (3) Scalar                  (3) Vector    (3) One or more Vectors 
  |    (4) Scalar                                (4) String 
  |  
  |  
  | (5) igeneric_function III 
  | In this section we will discuss an extension of the  igeneric_function 
  | interface that will allow for the overloading of a user defined custom 
  | function, where by it can return either a scalar or string value  type 
  | depending on the input parameter  sequence with which the function  is 
  | invoked. 
  |  
  |    template <typename T> 
  |    struct foo final : public exprtk::igeneric_function<T> 
  |    { 
  |       typedef typename exprtk::igeneric_function<T>::parameter_list_t 
  |                                                      parameter_list_t; 
  |  
  |       foo() 
  |       : exprtk::igeneric_function<T> 
  |         ( 
  |           "T:T|S:TS", 
  |           igfun_t::e_rtrn_overload 
  |         ) 
  |       {} 
  |  
  |       // Scalar value returning invocations 
  |       inline T operator()(const std::size_t& ps_index, 
  |                           parameter_list_t parameters) override 
  |       { 
  |          ... 
  |       } 
  |  
  |       // String value returning invocations 
  |       inline T operator()(const std::size_t& ps_index, 
  |                           std::string& result, 
  |                           parameter_list_t& parameters) override 
  |       { 
  |          ... 
  |       } 
  |    }; 
  |  
  |  
  | In the  example above  the custom  user defined  function "foo" can be 
  | invoked by using  either one of  two input parameter  sequences, which 
  | are defined as follows: 
  |  
  |    Sequence-0    Sequence-1 
  |    'T' -> T      'TS' -> S 
  |    (1) Scalar    (1) Scalar 
  |                  (2) String 
  |  
  |  
  | The parameter  sequence definitions  are identical  to the  previously 
  | defined igeneric_function, with the exception of the inclusion  of the 
  | return type - which can only be either a scalar T or a string S. 
  |  
  |  
  | (6) function_compositor 
  | The function  compositor is  a factory  that allows  one to define and 
  | construct a function using ExprTk syntax. The functions are limited to 
  | returning a single scalar value and consuming up to six parameters  as 
  | input. 
  |  
  | All composited functions are registered with a symbol table,  allowing 
  | them  to  call  other  functions  and  use  variables  that  have been 
  | registered with the symbol  table instance. Furthermore the  functions 
  | can be  recursive in  nature due  to the  inherent function  prototype 
  | forwarding  that  occurs during  construction.  The following  example 
  | defines,  by using  two different  methods, composited  functions and 
  | implicitly registering the functions with the denoted symbol table. 
  |  
  |    typedef exprtk::symbol_table<T>         symbol_table_t; 
  |    typedef exprtk::function_compositor<T>  compositor_t; 
  |    typedef typename compositor_t::function function_t; 
  |  
  |    T avogadro = T(6.022e23); 
  |  
  |    symbol_table_t symbol_table; 
  |  
  |    symbol_table.add_constant("avogadro", avogadro); 
  |  
  |    compositor_t compositor(symbol_table); 
  |  
  |    // Define function koo0(v1, v2) { ... } 
  |    compositor.add( 
  |       function_t("koo0"), 
  |       .vars("v1", "v2") 
  |       .expression 
  |       ( 
  |          " 1 + cos(v1 * v2) / avogadro; " 
  |       )); 
  |  
  |    // Define function koo1(x, y, z) { ... } 
  |    compositor.add( 
  |       function_t() 
  |       .name("koo1") 
  |       .var("x").var("y").var("z") 
  |       .expression 
  |       ( 
  |          "1 + koo0(x * y, 3) / z;" 
  |       )); 
  |  
  |  
  | A function compositor can also be instantiated without a symbol_table. 
  | When this is the case an internal symbol_table is used for holding the 
  | references to the composited functions. 
  |  
  |    compositor_t compositor; 
  |  
  |    // Define function koo2(v1, v2) { ... } 
  |    compositor.add( 
  |       function_t("koo2"), 
  |       .vars("v1", "v2", "v3") 
  |       .expression 
  |       ( " abs(v1 * v2) / v3; " )); 
  |  
  |  
  | When wanting to  reference functions from  the compositor above  in an 
  | expression, the compositor's symbol_table  will need to be  registered 
  | with the expression  prior to compilation,  as is demonstrated  in the 
  | following code: 
  |  
  |    expression_t expression; 
  |    . 
  |    . 
  |    expression.register_symbol_table(compositor.symbol_table()); 
  |  
  |  
  | In the situation where  more than one symbol table's contents will  be 
  | required by the functions  being composited, then those  symbol tables 
  | can be registered as auxiliary symbol tables with the compositor: 
  |  
  |    symbol_table_t global_symbol_table; 
  |    symbol_table_t local_symbol_table; 
  |    . 
  |    . 
  |    . 
  |    compositor_t compositor; 
  |  
  |    compositor.add_auxiliary_symtab(global_symbol_table); 
  |    compositor.add_auxiliary_symtab(local_symbol_table ); 
  |  
  | Note19: In the event, that two or more symbol tables contain similarly 
  | named  variables,  vectors,  strings   or  functions,  the  order   of 
  | registration with the compositor shall determine the symbol table from 
  | which the target symbol will be referenced. 
  |  
  |  
  | (7) Using Functions In Expressions 
  | For the above denoted custom and composited functions to be used in an 
  | expression, an instance of each function needs to be registered with a 
  | symbol_table that  has been  associated with  the expression instance. 
  | The following demonstrates how all the pieces are put together: 
  |  
  |    typedef exprtk::symbol_table<double>        symbol_table_t; 
  |    typedef exprtk::expression<double>          expression_t; 
  |    typedef exprtk::parser<double>              parser_t; 
  |    typedef exprtk::function_compositor<double> compositor_t; 
  |    typedef typename compositor_t::function     function_t; 
  |  
  |    foo<double> f; 
  |    boo<double> b; 
  |    too<double> t; 
  |    toupper<double> tu; 
  |  
  |    symbol_table_t symbol_table; 
  |    compositor_t   compositor(symbol_table); 
  |  
  |    symbol_table.add_function("foo",f); 
  |    symbol_table.add_function("boo",b); 
  |    symbol_table.add_function("too",t); 
  |  
  |    symbol_table 
  |       .add_function("toupper", tu, symbol_table_t::e_ft_strfunc); 
  |  
  |    compositor.add( 
  |       function_t("koo") 
  |       .var("v1") 
  |       .var("v2") 
  |       .expression 
  |       ( 
  |          "1 + cos(v1 * v2) / 3;" 
  |       )); 
  |  
  |    expression_t expression; 
  |    expression.register_symbol_table(symbol_table); 
  |  
  |    const std::string expression_str = 
  |       " if (foo(1,2,3) + boo(1) > boo(1/2, 2/3, 3/4, 4/5)) " 
  |       "    koo(3,4);                                       " 
  |       " else                                               " 
  |       "    too(2 * v1 + v2 / 3, 'abcdef'[2:4], 3.3);       " 
  |       "                                                    " 
  |  
  |    parser_t parser; 
  |    parser.compile(expression_str,expression); 
  |  
  |    expression.value(); 
  |  
  |  
  | (8) Function Side-Effects 
  | All function calls are assumed  to have side-effects by default.  This 
  | assumption implicitly disables constant folding optimisations when all 
  | parameters being passed to the function are deduced as being constants 
  | at compile time. 
  |  
  | If it is certain that the function being registered does not have  any 
  | side-effects and can  be correctly constant folded  where appropriate, 
  | then during the construction of the function the side-effect trait  of 
  | the function can be disabled. 
  |  
  |    template <typename T> 
  |    struct foo final : public exprtk::ifunction<T> 
  |    { 
  |       foo() : exprtk::ifunction<T>(3) 
  |       { 
  |          exprtk::disable_has_side_effects(*this); 
  |       } 
  |  
  |       T operator()(const T& v1, const T& v2, const T& v3) override 
  |       { ... } 
  |    }; 
  |  
  |  
  | (9) Zero Parameter Functions 
  | When  either  an  ifunction,  ivararg_function  or   igeneric_function 
  | derived type is defined with zero number of parameters, there are  two 
  | calling  conventions  within  expressions  that  are  allowed.  For  a 
  | function named 'foo' with zero input parameters the calling styles are 
  | as follows: 
  |  
  |    (1)  x + sin(foo()- 2) / y 
  |    (2)  x + sin(foo  - 2) / y 
  |  
  |  
  | By default the  zero parameter trait  is disabled. In  order to enable 
  | it, a process similar to that of  enabling of the side-effect trait is 
  | carried out: 
  |  
  |    template <typename T> 
  |    struct foo final : public exprtk::ivararg_function<T> 
  |    { 
  |       foo() 
  |       { 
  |          exprtk::enable_zero_parameters(*this); 
  |       } 
  |  
  |       inline T operator()(const std::vector<T>& arglist) override 
  |       { ... } 
  |    }; 
  |  
  |  
  | Note20: For the igeneric_function  type, there  also needs to be a 'Z' 
  | parameter sequence  defined in order for the  zero parameter  trait to 
  | properly take effect otherwise a compilation error will occur. 
  |  
  |  
  | (10) Free Functions 
  | The ExprTk symbol  table supports the  registration of free  functions 
  | and lambdas  (anonymous functors)  for use  in expressions.  The basic 
  | requirements  are similar  to those  found in  ifunction derived  user 
  | defined  functions. This  includes  support  for free  functions using 
  | anywhere from zero up to fifteen input parameters of scalar type, with 
  | a return type that is also scalar. Furthermore such functions will  by 
  | default be assumed to have side-effects and hence will not participate 
  | in constant folding optimisations. 
  |  
  | In the following  example, a one  input parameter free  function named 
  | 'compute1',  a  two  input  parameter  template  free  function  named 
  | 'compute2' and a three input parameter lambda named 'compute3' will be 
  | registered with the given symbol_table instance: 
  |  
  |    double compute1(double v0) 
  |    { 
  |       return 2.0 * std::abs(v0); 
  |    } 
  |  
  |    template <typename T> 
  |    T compute2(T v0, T v1) 
  |    { 
  |       return 2.0 * v0 + v1 / 3.0; 
  |    } 
  |    . 
  |    . 
  |    . 
  |  
  |    typedef exprtk::symbol_table<double> symbol_table_t; 
  |  
  |    symbol_table_t symbol_table; 
  |  
  |    symbol_table.add_function("compute1", compute1); 
  |    symbol_table.add_function("compute2", compute2<double>); 
  |  
  |    symbol_table.add_function( 
  |       "compute3", 
  |       [](double v0, double v1, double v2) -> double 
  |       { return v0 / v1 + v2; }); 
  |  
  |  
  | Note21: Similar to  variables registered with  symbol_table instances, 
  | for any of the following function providers: 
  |  
  |    1. ifunction 
  |    2. ivararg_function 
  |    3. igeneric_function 
  |    4. function_compositor 
  |    5. Free function 
  |    7. Lambda 
  |  
  |  
  | Their instance lifetimes must exceed the symbol_tables and expressions 
  | they are  registered with.  In the  event that  is not  the case,  the 
  | expected result shall be undefined behaviour. 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 16 - EXPRESSION DEPENDENTS] 
  | Any  expression  that  is  not  a  literal  (aka  constant)  will have 
  | dependencies. The types of  'dependencies' an expression can  have are 
  | as follows: 
  |  
  |    (a) Variables 
  |    (b) Vectors 
  |    (c) Strings 
  |    (d) Functions 
  |    (e) Assignments 
  |  
  |  
  | In  the  following  example the  denoted  expression  has its  various 
  | dependencies listed: 
  |  
  |    z := abs(x + sin(2 * pi / y)) 
  |  
  |    (a) Variables:   x, y, z and pi 
  |    (b) Functions:   abs, sin 
  |    (c) Assignments: z 
  |  
  |  
  | ExprTk allows for  the derivation of  expression dependencies via  the 
  | 'dependent_entity_collector'  (DEC).  When  activated  either  through 
  | 'compile_options' at the construction  of the parser or  through calls 
  | to enabler methods just prior to compilation, the DEC will proceed  to 
  | collect any  of the  relevant types  that are  encountered during  the 
  | parsing  phase.   Once  the   compilation  process   has  successfully 
  | completed, the  caller can  then obtain  a list  of symbols  and their 
  | associated types from the DEC. 
  |  
  | The kinds of  questions one can  ask regarding the  dependent entities 
  | within an expression are as follows: 
  |  
  |   * What user defined variables, vectors or strings are used? 
  |   * What functions or custom user functions are used? 
  |   * Which variables, vectors or strings have values assigned to them? 
  |  
  |  
  | The following example demonstrates usage of the DEC in determining the 
  | dependents of the given expression: 
  |  
  |    typedef typename parser_t:: 
  |       dependent_entity_collector::symbol_t symbol_t; 
  |  
  |    const std::string expression_string = 
  |       "z := abs(x + sin(2 * pi / y))" 
  |  
  |    T x,y,z; 
  |  
  |    parser_t parser; 
  |    symbol_table_t symbol_table; 
  |  
  |    symbol_table.add_variable("x",x); 
  |    symbol_table.add_variable("y",y); 
  |    symbol_table.add_variable("z",z); 
  |  
  |    expression_t expression; 
  |    expression.register_symbol_table(symbol_table); 
  |  
  |    // Collect only variable and function symbols 
  |    parser.dec().collect_variables() = true; 
  |    parser.dec().collect_functions() = true; 
  |  
  |    if (!parser.compile(expression_string,expression)) 
  |    { 
  |       // error.... 
  |    } 
  |  
  |    std::deque<symbol_t> symbol_list; 
  |  
  |    parser.dec().symbols(symbol_list); 
  |  
  |    for (std::size_t i = 0; i < symbol_list.size(); ++i) 
  |    { 
  |       const symbol_t& symbol = symbol_list[i]; 
  |  
  |       switch (symbol.second) 
  |       { 
  |          case parser_t::e_st_variable : ... break; 
  |          case parser_t::e_st_vector   : ... break; 
  |          case parser_t::e_st_string   : ... break; 
  |          case parser_t::e_st_function : ... break; 
  |       } 
  |    } 
  |  
  |  
  | Note22: The 'symbol_t'  type is a  std::pair comprising of  the symbol 
  | name (std::string) and the associated type of the symbol as denoted by 
  | the cases in the switch statement. 
  |  
  | Having  particular  symbols  (variable  or  function)  present  in  an 
  | expression is one form of dependency. Another and just as  interesting 
  | and important type of  dependency is that of  assignments. Assignments 
  | are the set of dependent symbols that 'may' have their values modified 
  | within an expression. The following are example expressions and  their 
  | associated assignments: 
  |  
  |        Assignments   Expression 
  |    (1) x             x := y + z 
  |    (2) x, y          x += y += z 
  |    (3) x, y, z       x := y += sin(z := w + 2) 
  |    (4) w, z          if (x > y, z := x + 2, w := 'A String') 
  |    (5) None          x + y + z 
  |  
  |  
  | Note23: In  expression 4,  both variables  'w' and  'z' are denoted as 
  | being assignments even though only one of them can ever be modified at 
  | the time of evaluation. Furthermore the determination of which of  the 
  | two variables the modification will occur upon can only be known  with 
  | certainty at evaluation time and not beforehand, hence both are listed 
  | as being candidates for assignment. 
  |  
  | The following builds upon the previous example demonstrating the usage 
  | of the DEC in determining the 'assignments' of the given expression: 
  |  
  |    // Collect assignments 
  |    parser.dec().collect_assignments() = true; 
  |  
  |    if (!parser.compile(expression_string,expression)) 
  |    { 
  |       // error.... 
  |    } 
  |  
  |    std::deque<symbol_t> symbol_list; 
  |  
  |    parser.dec().assignment_symbols(symbol_list); 
  |  
  |    for (std::size_t i = 0; i < symbol_list.size(); ++i) 
  |    { 
  |       symbol_t& symbol = symbol_list[i]; 
  |  
  |       switch (symbol.second) 
  |       { 
  |          case parser_t::e_st_variable : ... break; 
  |          case parser_t::e_st_vector   : ... break; 
  |          case parser_t::e_st_string   : ... break; 
  |       } 
  |    } 
  |  
  |  
  | Note24: The  assignments will  only consist  of variable  types and as 
  | such will not contain symbols denoting functions. 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 17 - HIERARCHIES OF SYMBOL TABLES] 
  | Most situations will only require a single symbol_table instance to be 
  | associated with a given expression instance. 
  |  
  | However as an expression can have more than one symbol table  instance 
  | associated  with  itself,  when  building  more  complex  systems that 
  | utilise many expressions  where each can  in turn utilise  one or more 
  | variables  from  a  large set  of  potential  variables, functions  or 
  | constants, it becomes evident  that grouping variables into  layers of 
  | symbol_tables will simplify and streamline the overall process. 
  |  
  | A recommended hierarchy of symbol tables is the following: 
  |  
  |    (a) Global constant value symbol table 
  |    (b) Global non side-effect functions symbol table 
  |    (c) Global variable symbol table 
  |    (d) Expression specific variable symbol table 
  |  
  |  
  | (a) Global constant value symbol table 
  | This symbol table will  contain constant variables denoting  immutable 
  | values. These variables can be made available to all expressions,  and 
  | in turn expressions  will assume the  values themselves will  never be 
  | modified for the  duration of the  process run-time. Examples  of such 
  | variables are: 
  |  
  |    (1) pi or e 
  |    (2) speed_of_light 
  |    (3) avogadro_number 
  |    (4) num_cpus 
  |  
  |  
  | (b) Global non side-effect functions symbol table 
  | This symbol table will contain  only user defined functions that  will 
  | not  incur  any  side-effects  that  are  observable  to  any  of  the 
  | expressions that invoke them. These functions shall be  thread-safe or 
  | threading invariant and  will not maintain  any form of  state between 
  | invocations. Examples of such functions are: 
  |  
  |    (1) calc_volume_of_sphere(r) 
  |    (2) distance(x0,y0,x1,y1) 
  |  
  |  
  | (c) Global variable symbol table 
  | This symbol table  will contain variables  that will be  accessible to 
  | all associated expressions  and will not  be specific or  exclusive to 
  | any one expression. This variant  differs from (a) in that  the values 
  | of the  variables can  change (or  be updated)  between evaluations of 
  | expressions  -   but  through   properly  scheduled   evaluations  are 
  | guaranteed  to never  change during  the evaluation  of any  dependent 
  | expressions. Furthermore it  is assumed that  these variables will  be 
  | used in a  read-only context and  that no expressions  will attempt to 
  | modify these variables via assignments or other means. 
  |  
  |    (1) price_of_stock_xyz 
  |    (2) outside_temperature or inside_temperature 
  |    (3) fuel_in_tank 
  |    (4) num_customers_in_store 
  |    (5) num_items_on_shelf 
  |  
  |  
  | (d) Expression specific variable symbol table 
  | This  symbol_table  is  the most  common form,  and is  used to  store 
  | variables that are specific and exclusive to a particular  expression. 
  | That is to say references  to variables in this symbol_table  will not 
  | be  part of  another expression.  Though it  may be  possible to  have 
  | expressions that  contain the  variables with  the same  name, in that 
  | case those variables will be distinctly different. Which would mean if 
  | a particular  expression were  to be  compiled twice,  each expression 
  | would have its  own unique symbol_table  which in turn  would have its 
  | own instances of those variables. Examples of such variables could be: 
  |  
  |    (1) x or y 
  |    (2) customer_name 
  |  
  |  
  | The following is a diagram depicting a possible variant of the denoted 
  | symbol  table  hierarchies.  In  the  diagram  there  are  two  unique 
  | expressions, each of  which have a  reference to the  Global constant, 
  | functions and variables symbol tables and an exclusive reference to  a 
  | local symbol table. 
  |  
  |   +-------------------------+    +-------------------------+ 
  |   |     Global Constants    |    |     Global Functions    | 
  |   |       Symbol Table      |    |       Symbol Table      | 
  |   +----o--o-----------------+    +--------------------o----+ 
  |        |  |                                           | 
  |        |  |                                           +-------+ 
  |        |  +------------------->----------------------------+  | 
  |        |         +----------------------------+            |  | 
  |        |         |      Global Variables      |            |  | 
  |        |  +------o        Symbol Table        o-----+      |  V 
  |        |  |      +----------------------------+     |      |  | 
  |        |  |                                         |      |  | 
  |        |  | +----------------+   +----------------+ |      |  | 
  |        |  | | Symbol Table 0 |   | Symbol Table 1 | |      V  | 
  |        |  | +--o-------------+   +--o-------------+ |      |  | 
  |        |  |    |                    |               |      |  | 
  |        |  |    |                    |               |      |  | 
  |     +--V--V----V---------+        +-V---------------V--+   |  | 
  |     |    Expression 0    |        |    Expression 1    |<--+--+ 
  |     |  '2 * sin(x) - y'  |        |  'k + abs(x - y)'  | 
  |     +--------------------+        +--------------------+ 
  |  
  |  
  | Bringing  all of  the above  together, in  the following  example the 
  | hierarchy  of  symbol  tables  are instantiated  and  initialised. An 
  | expression that makes use of various elements of each symbol table is 
  | then compiled and later on evaluated: 
  |  
  |    typedef exprtk::symbol_table<double> symbol_table_t; 
  |    typedef exprtk::expression<double>   expression_t; 
  |  
  |    // Setup global constants symbol table 
  |    symbol_table_t glbl_const_symbol_table; 
  |    glbl_const_symbtab.add_constants(); // pi, epsilon and inf 
  |    glbl_const_symbtab.add_constant("speed_of_light",299e6); 
  |    glbl_const_symbtab.add_constant("avogadro_number",6e23); 
  |  
  |    // Setup global function symbol table 
  |    symbol_table_t glbl_funcs_symbol_table; 
  |    glbl_func_symbtab.add_function('distance',distance); 
  |    glbl_func_symbtab.add_function('calc_spherevol',calc_sphrvol); 
  |  
  |    ...... 
  |  
  |    // Setup global variable symbol table 
  |    symbol_table_t glbl_variable_symbol_table; 
  |    glbl_variable_symbtab.add_variable('temp_outside',thermo.outside); 
  |    glbl_variable_symbtab.add_variable('temp_inside' ,thermo.inside ); 
  |    glbl_variable_symbtab.add_variable('num_cstmrs',store.num_cstmrs); 
  |  
  |    ...... 
  |  
  |    double x,y,z; 
  |  
  |    // Setup expression specific symbol table 
  |    symbol_table_t symbol_table; 
  |    symbol_table.add_variable('x',x); 
  |    symbol_table.add_variable('y',y); 
  |    symbol_table.add_variable('z',z); 
  |  
  |    expression_t expression; 
  |  
  |    // Register the various symbol tables 
  |    expression 
  |       .register_symbol_table(symbol_table); 
  |  
  |    expression 
  |       .register_symbol_table(glbl_funcs_symbol_table); 
  |  
  |    expression 
  |       .register_symbol_table(glbl_const_symbol_table); 
  |  
  |    expression 
  |       .register_symbol_table(glbl_variable_symbol_table); 
  |  
  |    const std::string expression_str = 
  |       "abs(temp_inside - temp_outside) + 2 * speed_of_light / x" 
  |  
  |    parser_t parser; 
  |    parser.compile(expression_str,expression); 
  |  
  |    ...... 
  |  
  |    while (keep_evaluating) 
  |    { 
  |      .... 
  |  
  |      T result = expression.value(); 
  |  
  |      .... 
  |    } 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 18 - UNKNOWN UNKNOWNS] 
  | In this section  we will discuss  the process of  handling expressions 
  | with a mix of known and unknown variables. Initially a discussion into 
  | the types of expressions that exist will be provided, then a series of 
  | possible solutions will be presented for each scenario. 
  |  
  | When parsing an expression, there  may be situations where one  is not 
  | fully  aware  of what  if  any variables  will  be used  prior  to the 
  | expression being compiled. 
  |  
  | This can become problematic, as in the default scenario it is  assumed 
  | the symbol_table that is registered with the expression instance  will 
  | already possess  the  externally  available  variables,  functions and 
  | constants needed during the compilation of the expression. 
  |  
  | In the event there are symbols in the expression that can't be  mapped 
  | to   either   a  reserved   word,   or  located   in   the  associated 
  | symbol_table(s), an "Undefined  symbol" error will  be raised and  the 
  | compilation process will fail. 
  |  
  | The numerous  scenarios that  can occur  when compiling  an expression 
  | with ExprTk generally fall into one of the following three categories: 
  |  
  |    (a) No external variables 
  |    (b) Predetermined set of external variables 
  |    (c) Unknown set of variables 
  |  
  |  
  | (a) No external variables 
  | These  are  expressions that  contain  no external  variables  but may 
  | contain  local  variables.  As  local  variables  cannot  be  accessed 
  | externally from the  expression, it is  assumed that such  expressions 
  | will not have  a need for  a symbol_table and  furthermore expressions 
  | which  don't  make  use of  functions that  have side-effects  will be 
  | evaluated completely at  compile time resulting  in a constant  return 
  | value. The following are examples of such expressions: 
  |  
  |    (1) 1 + 2 
  |    (2) var x := 3; 2 * x - 3 
  |    (3) var x := 3; var y := abs(x - 8); x - y / 7 
  |  
  |  
  | (b) Predetermined set of external variables 
  | These  are  expressions  that are  comprised  of  externally available 
  | variables  and functions  and will  only compile  successfully if  the 
  | symbols that  correspond to  the variables  and functions  are already 
  | defined in their associated symbol_table(s).  This is by far the  most 
  | common scenario when using ExprTk. 
  |  
  | As an example, one may have three external variables: x, y and z which 
  | have been registered with  the associated symbol_table, and  will then 
  | need to compile  and evaluate expressions  comprised of any  subset of 
  | these  three  variables. The  following  are a  few  examples of  such 
  | expressions: 
  |  
  |    (1) 1 + x 
  |    (2) x / y 
  |    (3) 2 * x * y / z 
  |  
  |  
  | In  this  scenario   one  can  use   the  'dependent_entity_collector' 
  | component as described in [Section  16] to further determine which  of 
  | the registered variables were  actually used in the  given expression. 
  | As  an example  once the  set of  utilised  variables  are known,  any 
  | further 'attention'  can be  restricted to  only those  variables when 
  | evaluating the expression. This can be quite useful when dealing  with 
  | expressions that can draw from a set of hundreds or even thousands  of 
  | variables. 
  |  
  |  
  | (c) Unknown set of variables 
  | These are  expressions that  are comprised  of symbols  other than the 
  | standard ExprTk reserved words or what has been registered with  their 
  | associated symbol_table, and will normally fail compilation due to the 
  | associated symbol_table not having a  reference to them. As such  this 
  | scenario can be  seen as a  combination of scenario  B, where one  may 
  | have a symbol_table with registered variables, but would also like  to 
  | handle  the  situation  of  variables  that  aren't  present  in  said 
  | symbol_table. 
  |  
  | When dealing with expressions of category (c), one must perform all of 
  | the following: 
  |  
  |    (1) Determine the variables used in the expression 
  |    (2) Populate a symbol_table(s) with the entities from (1) 
  |    (3) Compile the expression 
  |    (4) Provide a means by which the entities from (1) can be modified 
  |  
  |  
  | Depending on the nature of processing,  steps (1) and (2) can be  done 
  | either independently of each other or combined into one. The following 
  | example  will  initially  look  at  solving  the  problem  of  unknown 
  | variables with the  latter method using  the 'unknown_symbol_resolver' 
  | component. 
  |  
  |    typedef exprtk::symbol_table<T> symbol_table_t; 
  |    typedef exprtk::expression<T>   expression_t; 
  |    typedef exprtk::parser<T>       parser_t; 
  |  
  |    T x = T(123.456); 
  |    T y = T(789.123); 
  |  
  |    symbol_table_t unknown_var_symbol_table; 
  |  
  |    symbol_table_t symbol_table; 
  |    symbol_table.add_variable("x",x); 
  |    symbol_table.add_variable("y",y); 
  |  
  |    expression_t expression; 
  |    expression.register_symbol_table(unknown_var_symbol_table); 
  |    expression.register_symbol_table(symbol_table); 
  |  
  |    parser_t parser; 
  |    parser.enable_unknown_symbol_resolver(); 
  |  
  |    const std::string expression_str = "x + abs(y / 3k) * z + 2" 
  |  
  |    parser.compile(expression_str,expression); 
  |  
  |  
  | In the  example above,  the symbols  'k' and  'z' will  be treated  as 
  | unknown symbols. The  parser in the  example is set  to handle unknown 
  | symbols using the built-in default unknown_symbol_resolver (USR).  The 
  | default  USR  will  automatically resolve  any  unknown  symbols as  a 
  | variable (scalar type). The new variables will be added to the primary 
  | symbol_table,  which in  this case  is the  'unknown_var_symbol_table' 
  | instance.  Once  the  compilation  has  completed  successfully,   the 
  | variables that were resolved  during compilation can be  accessed from 
  | the   primary   symbol_table   using   the   'get_variable_list'   and 
  | 'variable_ref' methods and then if needed can be modified  accordingly 
  | after which the expression itself can be evaluated. 
  |  
  |    std::vector<std::string> variable_list; 
  |  
  |    unknown_var_symbol_table.get_variable_list(variable_list); 
  |  
  |    for (const auto& var_name : variable_list) 
  |    { 
  |       T& v = unknown_var_symbol_table.variable_ref(var_name); 
  |  
  |       v = ...; 
  |    } 
  |  
  |    ... 
  |  
  |    expression.value(); 
  |  
  |  
  | Note25: As previously  mentioned the  default  USR  will automatically 
  | assume any unknown symbol to be a valid scalar variable, and will then 
  | proceed to add said symbol  as a variable to the  primary symbol_table 
  | of the associated expression during the compilation process. However a 
  | problem that may arise, is  that expressions that are parsed  with the 
  | USR enabled,  but contain  'typos' or  otherwise syntactic  errors may 
  | inadvertently compile successfully due to the simplistic nature of the 
  | default USR. The following are some example expressions: 
  |  
  |    (1) 1 + abz(x + 1) 
  |    (2) sine(y / 2) - coz(3x) 
  |  
  |  
  | The two  expressions above contain misspelt  symbols (abz,  sine, coz) 
  | which if implied  multiplications and default  USR are enabled  during 
  | compilation will result in them being assumed to be valid 'variables', 
  | which obviously is  not the intended  outcome by the  user. A possible 
  | solution to this  problem is for  one to implement  their own specific 
  | USR that will perform a user defined business logic in determining  if 
  | an encountered unknown symbol should be treated as a variable or if it 
  | should raise a compilation error. The following example demonstrates a 
  | simple user defined USR: 
  |  
  |    typedef exprtk::symbol_table<T> symbol_table_t; 
  |    typedef exprtk::expression<T>   expression_t; 
  |    typedef exprtk::parser<T>       parser_t; 
  |  
  |    template <typename T> 
  |    struct my_usr final : public parser_t::unknown_symbol_resolver 
  |    { 
  |       typedef typename parser_t::unknown_symbol_resolver usr_t; 
  |  
  |       bool process(const std::string& unknown_symbol, 
  |                    typename usr_t::usr_symbol_type& st, 
  |                    T& default_value, 
  |                    std::string& error_message) override 
  |       { 
  |          if (0 != unknown_symbol.find("var_")) 
  |          { 
  |             error_message = "Invalid symbol: " + unknown_symbol; 
  |             return false; 
  |          } 
  |  
  |          st = usr_t::e_usr_variable_type; 
  |          default_value = T(123.123); 
  |  
  |          return true; 
  |       } 
  |    }; 
  |  
  |    ... 
  |  
  |    T x = T(123.456); 
  |    T y = T(789.123); 
  |  
  |    symbol_table_t unknown_var_symbol_table; 
  |  
  |    symbol_table_t symbol_table; 
  |    symbol_table.add_variable("x",x); 
  |    symbol_table.add_variable("y",y); 
  |  
  |    expression_t expression; 
  |    expression.register_symbol_table(unknown_var_symbol_table); 
  |    expression.register_symbol_table(symbol_table); 
  |  
  |    my_usr<T> musr; 
  |  
  |    parser_t parser; 
  |    parser.enable_unknown_symbol_resolver(&musr); 
  |  
  |    std::string expression_str = "var_x + abs(var_y - 3) * var_z" 
  |  
  |    parser.compile(expression_str,expression); 
  |  
  |  
  | In  the  example  above,  a user  specified  USR  is  defined, and  is 
  | registered   with   the  parser   enabling   the  USR   functionality. 
  | Subsequently during the compilation process when an unknown symbol  is 
  | encountered, the USR's process method will be invoked. The USR in  the 
  | example  will only  'accept' unknown  symbols  that  have a  prefix of 
  | 'var_' as being valid variables, all other unknown symbols will result 
  | in a compilation error being raised. 
  |  
  | In the example above  the callback of the  USR that is invoked  during 
  | the unknown symbol resolution process only allows for scalar variables 
  | to be defined and resolved -  as that is the simplest and  most common 
  | form. 
  |  
  | There is a further  extended  version  of  the  callback  that  can be 
  | overridden that will allow for  more control and choice over  the type 
  | of symbol being  resolved. The following  is an example  definition of 
  | said extended callback: 
  |  
  |    template <typename T> 
  |    struct my_usr final : public parser_t::unknown_symbol_resolver 
  |    { 
  |      typedef typename parser_t::unknown_symbol_resolver usr_t; 
  |  
  |      my_usr() 
  |      : usr_t(usr_t::e_usrmode_extended) 
  |      {} 
  |  
  |      bool process(const std::string& unknown_symbol, 
  |                   symbol_table_t&    symbol_table, 
  |                   std::string&       error_message) override 
  |      { 
  |         bool result = false; 
  |  
  |         if (0 == unknown_symbol.find("var_")) 
  |         { 
  |            // Default value of zero 
  |            result = symbol_table.create_variable(unknown_symbol,0); 
  |  
  |            if (!result) 
  |            { 
  |               error_message = "Failed to create variable..." 
  |            } 
  |         } 
  |         else if (0 == unknown_symbol.find("str_")) 
  |         { 
  |            // Default value of empty string 
  |            result = symbol_table.create_stringvar(unknown_symbol,""); 
  |  
  |            if (!result) 
  |            { 
  |               error_message = "Failed to create string variable..." 
  |            } 
  |         } 
  |         else 
  |            error_message = "Indeterminable symbol type." 
  |  
  |         return result; 
  |      } 
  |    }; 
  |  
  |  
  | In the  example above,  the USR  callback when  invoked will  pass the 
  | primary symbol table associated with the expression being parsed.  The 
  | symbol  resolution  business  logic  can  then  determine  under  what 
  | conditions  a  symbol will  be  resolved including  its  type (scalar, 
  | string, vector etc) and default value. When the callback  successfully 
  | returns  the  symbol  parsing and  resolution  process  will again  be 
  | executed by the parser. The idea here is that given the primary symbol 
  | table will now have the previously detected unknown symbol registered, 
  | it will be correctly resolved  and the general parsing processing  can 
  | then resume as per normal. 
  |  
  | Note26: In order to have  the USR's extended mode callback  be invoked 
  | it is necessary to pass  the e_usrmode_extended enum value during  the 
  | constructor of the user defined USR. 
  |  
  | Note27: The primary symbol table for an expression is the first symbol 
  | table to be registered with that instance of the expression. 
  |  
  | Note28: For a successful symbol resolution using the normal USR all of 
  | the following are required: 
  |  
  |    (1) Only if successful shall the process method return TRUE 
  |    (2) The default_value parameter will have been set 
  |    (3) The error_message parameter will be empty 
  |    (4) usr_symbol_type input parameter field will be set to either: 
  |          (*) e_usr_variable_type 
  |          (*) e_usr_constant_type 
  |  
  | Note29: For a successful symbol resolution using the extended USR  all 
  | of the following are required: 
  |  
  |    (1) Only if successful shall the process method return TRUE 
  |    (2) symbol_table parameter will have had the newly resolved 
  |        variable or string added to it 
  |    (3) error_message parameter will be empty 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 19 - ENABLING & DISABLING FEATURES] 
  | The parser can be configured via its settings instance to either allow 
  | or  disallow certain  features that  are available  within the  ExprTk 
  | grammar. The features fall  into one  of the following six categories: 
  |  
  |    (1) Base Functions 
  |    (2) Control Flow Structures 
  |    (3) Logical Operators 
  |    (4) Arithmetic Operators 
  |    (5) Inequality Operators 
  |    (6) Assignment Operators 
  |  
  |  
  | (1) Base Functions 
  | The list of available base functions is as follows: 
  |  
  |    abs, acos, acosh, asin,  asinh, atan, atanh, atan2,  avg, ceil, 
  |    clamp,  cos,  cosh, cot,  csc,  equal, erf,  erfc,  exp, expm1, 
  |    floor,  frac,  hypot,  iclamp, like,  log,  log10,  log2, logn, 
  |    log1p, mand, max, min, mod,  mor, mul, ncdf, pow, root,  round, 
  |    roundn, sec, sgn, sin, sinc, sinh, sqrt, sum, swap, tan,  tanh, 
  |    trunc, not_equal, inrange, deg2grad, deg2rad, rad2deg, grad2deg 
  |  
  |  
  | The above mentioned base functions  can be either enabled or  disabled 
  | 'all' at once, as is demonstrated below: 
  |  
  |    parser_t parser; 
  |    expression_t expression; 
  |  
  |    parser.settings().disable_all_base_functions(); 
  |  
  |    parser 
  |       .compile("2 * abs(2 - 3)",expression); // compilation failure 
  |  
  |    parser.settings().enable_all_base_functions(); 
  |  
  |    parser 
  |       .compile("2 * abs(2 - 3)",expression); // compilation success 
  |  
  |  
  | One can also enable or disable specific base functions. The  following 
  | example  demonstrates  the disabling  of  the trigonometric  functions 
  | 'sin' and 'cos': 
  |  
  |    parser_t parser; 
  |    expression_t expression; 
  |  
  |    parser.settings() 
  |       .disable_base_function(settings_t::e_bf_sin) 
  |       .disable_base_function(settings_t::e_bf_cos); 
  |  
  |    parser 
  |       .compile("(sin(x) / cos(x)) == tan(x)",expression); // failure 
  |  
  |    parser.settings() 
  |       .enable_base_function(settings_t::e_bf_sin) 
  |       .enable_base_function(settings_t::e_bf_cos); 
  |  
  |    parser 
  |       .compile("(sin(x) / cos(x)) == tan(x)",expression); // success 
  |  
  |  
  | (2) Control Flow Structures 
  | The list of available control flow structures is as follows: 
  |  
  |    (a) If or If-Else 
  |    (b) Switch statement 
  |    (c) For Loop 
  |    (d) While Loop 
  |    (e) Repeat Loop 
  |  
  |  
  | The  above  mentioned  control flow structures  can be  either enabled 
  | or disabled 'all' at once, as is demonstrated below: 
  |  
  |    parser_t parser; 
  |    expression_t expression; 
  |  
  |    const std::string program = 
  |       " var x := 0;                      " 
  |       " for (var i := 0; i < 10; i += 1) " 
  |       " {                                " 
  |       "   x += i;                        " 
  |       " }                                " 
  |  
  |    parser.settings().disable_all_control_structures(); 
  |  
  |    parser 
  |       .compile(program,expression); // compilation failure 
  |  
  |    parser.settings().enable_all_control_structures(); 
  |  
  |    parser 
  |       .compile(program,expression); // compilation success 
  |  
  |  
  | One can also enable or  disable specific control flow structures.  The 
  | following example demonstrates the  disabling of the for-loop  control 
  | flow structure: 
  |  
  |    parser_t parser; 
  |    expression_t expression; 
  |  
  |    const std::string program = 
  |       " var x := 0;                      " 
  |       " for (var i := 0; i < 10; i += 1) " 
  |       " {                                " 
  |       "   x += i;                        " 
  |       " }                                " 
  |  
  |    parser.settings() 
  |       .disable_control_structure(settings_t::e_ctrl_for_loop); 
  |  
  |    parser 
  |       .compile(program,expression); // failure 
  |  
  |    parser.settings() 
  |       .enable_control_structure(settings_t::e_ctrl_for_loop); 
  |  
  |    parser 
  |       .compile(program,expression); // success 
  |  
  |  
  | (3) Logical Operators 
  | The list of available logical operators is as follows: 
  |  
  |    and, nand, nor, not, or, xnor, xor, &, | 
  |  
  |  
  | The  above  mentioned  logical  operators  can  be  either  enabled or 
  | disabled 'all' at once, as is demonstrated below: 
  |  
  |    parser_t parser; 
  |    expression_t expression; 
  |  
  |    parser.settings().disable_all_logic_ops(); 
  |  
  |    parser 
  |       .compile("1 or not(0 and 1)",expression); // compilation failure 
  |  
  |    parser.settings().enable_all_logic_ops(); 
  |  
  |    parser 
  |       .compile("1 or not(0 and 1)",expression); // compilation success 
  |  
  |  
  | One  can  also  enable  or  disable  specific  logical  operators. The 
  | following  example  demonstrates  the disabling  of the  'and' logical 
  | operator: 
  |  
  |    parser_t parser; 
  |    expression_t expression; 
  |  
  |    parser.settings() 
  |       .disable_logic_operation(settings_t::e_logic_and); 
  |  
  |    parser 
  |       .compile("1 or not(0 and 1)",expression); // failure 
  |  
  |    parser.settings() 
  |       .enable_logic_operation(settings_t::e_logic_and); 
  |  
  |    parser 
  |       .compile("1 or not(0 and 1)",expression); // success 
  |  
  |  
  | (4) Arithmetic Operators 
  | The list of available arithmetic operators is as follows: 
  |  
  |    +, -, *, /, %, ^ 
  |  
  |  
  | The  above mentioned  arithmetic operators  can be  either enabled  or 
  | disabled 'all' at once, as is demonstrated below: 
  |  
  |    parser_t parser; 
  |    expression_t expression; 
  |  
  |    parser.settings().disable_all_arithmetic_ops(); 
  |  
  |    parser 
  |       .compile("1 + 2 / 3",expression); // compilation failure 
  |  
  |    parser.settings().enable_all_arithmetic_ops(); 
  |  
  |    parser 
  |       .compile("1 + 2 / 3",expression); // compilation success 
  |  
  |  
  | One  can also  enable or  disable specific  arithmetic operators.  The 
  | following  example  demonstrates  the disabling  of  the  addition '+' 
  | arithmetic operator: 
  |  
  |    parser_t parser; 
  |    expression_t expression; 
  |  
  |    parser.settings() 
  |       .disable_arithmetic_operation(settings_t::e_arith_add); 
  |  
  |    parser 
  |       .compile("1 + 2 / 3",expression); // failure 
  |  
  |    parser.settings() 
  |       .enable_arithmetic_operation(settings_t::e_arith_add); 
  |  
  |    parser 
  |       .compile("1 + 2 / 3",expression); // success 
  |  
  |  
  | (5) Inequality Operators 
  | The list of available inequality operators is as follows: 
  |  
  |    <, <=, >, >=, ==, =, != <> 
  |  
  |  
  | The  above mentioned  inequality operators  can be  either enabled  or 
  | disabled 'all' at once, as is demonstrated below: 
  |  
  |    parser_t parser; 
  |    expression_t expression; 
  |  
  |    parser.settings().disable_all_inequality_ops(); 
  |  
  |    parser 
  |       .compile("1 < 3",expression); // compilation failure 
  |  
  |    parser.settings().enable_all_inequality_ops(); 
  |  
  |    parser 
  |       .compile("1 < 3",expression); // compilation success 
  |  
  |  
  | One  can also  enable or  disable specific  inequality operators.  The 
  | following  example demonstrates  the disabling  of  the  less-than '<' 
  | inequality operator: 
  |  
  |    parser_t parser; 
  |    expression_t expression; 
  |  
  |    parser.settings() 
  |       .disable_inequality_operation(settings_t::e_ineq_lt); 
  |  
  |    parser 
  |       .compile("1 < 3",expression); // failure 
  |  
  |    parser.settings() 
  |       .enable_inequality_operation(settings_t::e_ineq_lt); 
  |  
  |    parser 
  |       .compile("1 < 3",expression); // success 
  |  
  |  
  | (6) Assignment Operators 
  | The list of available assignment operators is as follows: 
  |  
  |    :=, +=, -=, *=, /=, %= 
  |  
  |  
  | The  above mentioned  assignment operators  can be  either enabled  or 
  | disabled 'all' at once, as is demonstrated below: 
  |  
  |    T x = T(0); 
  |  
  |    parser_t       parser; 
  |    expression_t   expression; 
  |    symbol_table_t symbol_table; 
  |  
  |    symbol_table.add_variable("x",x); 
  |  
  |    expression.register_symbol_table(symbol_table); 
  |  
  |    parser.settings().disable_all_assignment_ops(); 
  |  
  |    parser 
  |       .compile("x := 3",expression); // compilation failure 
  |  
  |    parser.settings().enable_all_assignment_ops(); 
  |  
  |    parser 
  |       .compile("x := 3",expression); // compilation success 
  |  
  |  
  | One  can also  enable or  disable specific  assignment operators.  The 
  | following  example demonstrates  the  disabling  of the  '+=' addition 
  | assignment operator: 
  |  
  |    T x = T(0); 
  |  
  |    parser_t       parser; 
  |    expression_t   expression; 
  |    symbol_table_t symbol_table; 
  |  
  |    symbol_table.add_variable("x",x); 
  |  
  |    expression.register_symbol_table(symbol_table); 
  |  
  |    parser.settings() 
  |       .disable_assignment_operation(settings_t::e_assign_addass); 
  |  
  |    parser 
  |       .compile("x += 3",expression); // failure 
  |  
  |    parser.settings() 
  |       .enable_assignment_operation(settings_t::e_assign_addass); 
  |  
  |    parser 
  |       .compile("x += 3",expression); // success 
  |  
  |  
  | Note30: In the  event of  a  base  function  being  disabled, one  can 
  | redefine  the  base  function  using  the  standard  custom   function 
  | definition process.  In the  following example  the 'sin'  function is 
  | disabled then redefined as a function taking degree input. 
  |  
  |    template <typename T> 
  |    struct sine_deg final : public exprtk::ifunction<T> 
  |    { 
  |       sine_deg() : exprtk::ifunction<T>(1) {} 
  |  
  |       inline T operator()(const T& v) override 
  |       { 
  |          const T pi = exprtk::details::numeric::constant::pi; 
  |          return std::sin((v * T(pi)) / T(180)); 
  |       } 
  |    }; 
  |  
  |     ... 
  |  
  |    typedef exprtk::symbol_table<T> symbol_table_t; 
  |    typedef exprtk::expression<T>   expression_t; 
  |    typedef exprtk::parser<T>       parser_t; 
  |  
  |    typedef typename parser_t::settings_store settings_t; 
  |  
  |    sine_deg<T> sine; 
  |  
  |    symbol_table.add_reserved_function("sin",sine); 
  |  
  |    expression_t expression; 
  |  
  |    expression.register_symbol_table(symbol_table); 
  |  
  |    parser_t parser; 
  |  
  |    parser.settings() 
  |       .disable_base_function(settings_t::e_bf_sin); 
  |  
  |    parser.compile("1 + sin(30)",expression); 
  |  
  |  
  | In the example above, the custom 'sin' function is registered with the 
  | symbol_table using the method 'add_reserved_function'. This is done so 
  | as to bypass the checks for reserved words that are carried out on the 
  | provided symbol names when calling the standard 'add_function' method. 
  | Normally if  a user  specified symbol  name conflicts  with any of the 
  | ExprTk reserved words, the add_function call will fail. 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 20 - EXPRESSION RETURN VALUES] 
  | ExprTk expressions can return immediately from any point by  utilising 
  | the return call. Furthermore the  return call can be used  to transfer 
  | out multiple return values from within the expression. 
  |  
  | If an expression evaluation exits using a return point, the result  of 
  | the call  to the 'value' method will be  NaN, and  it is expected that 
  | the return values will be available from the results_context. 
  |  
  | In  the  following example  there  are  three  return  points  in  the 
  | expression.  If  neither  of  the  return  points  are  hit,  then the 
  | expression will return normally. 
  |  
  |    const std::string expression_string = 
  |       " if (x < y)                                   " 
  |       "    return [x + 1,'return-call 1'];           " 
  |       " else if (x > y)                              " 
  |       "    return [y / 2, y + 1, 'return-call 2'];   " 
  |       " else if (equal(x,y))                         " 
  |       "    x + y;                                    " 
  |       " return [x, y, x + y, x - y, 'return-call 3'] " 
  |  
  |    typedef exprtk::symbol_table<double> symbol_table_t; 
  |    typedef exprtk::expression<double>   expression_t; 
  |    typedef exprtk::parser<double>       parser_t; 
  |  
  |    symbol_table_t symbol_table; 
  |    expression_t   expression; 
  |    parser_t       parser; 
  |  
  |    double x = 0; 
  |    double y = 0; 
  |  
  |    symbol_table.add_variable("x",x); 
  |    symbol_table.add_variable("y",y); 
  |  
  |    expression.register_symbol_table(symbol_table); 
  |  
  |    parser.compile(expression_string,expression); 
  |  
  |    T result = expression.value(); 
  |  
  |    if (expression.return_invoked()) 
  |    { 
  |       typedef exprtk::results_context<T> results_context_t; 
  |       typedef typename results_context_t::type_store_t type_t; 
  |       typedef typename type_t::scalar_view scalar_t; 
  |       typedef typename type_t::vector_view vector_t; 
  |       typedef typename type_t::string_view string_t; 
  |  
  |       const results_context_t& results = expression.results(); 
  |  
  |       for (std::size_t i = 0; i < results.count(); ++i) 
  |       { 
  |          type_t t = results[i]; 
  |  
  |          switch (t.type) 
  |          { 
  |             case type_t::e_scalar : ... 
  |                                     break; 
  |  
  |             case type_t::e_vector : ... 
  |                                     break; 
  |  
  |             case type_t::e_string : ... 
  |                                     break; 
  |  
  |             default               : continue; 
  |          } 
  |    } 
  |  
  |  
  | In the above example, there are three possible "return" points and one 
  | regular result. Only one of the four paths can ever be realised. Hence 
  | it is necessary to capture  the result of the expression  value method 
  | call. In the event,  the call to return_invoked  is not true then  the 
  | non-return code  path was  executed and  the result  of the evaluation 
  | will be the result of the expression's value method. 
  |  
  | Note31: Processing of  the return  results is  similar to  that of the 
  | generic function call parameters. 
  |  
  | The results_context provides getter  methods for each of  the possible 
  | return types (scalar, vector and string) and can be used as follows: 
  |  
  |    typedef exprtk::symbol_table<T> symbol_table_t; 
  |    typedef exprtk::expression<T>   expression_t; 
  |    typedef exprtk::parser<T>       parser_t; 
  |  
  |    const std::string expression_str = 
  |       " if (x > y)                                  " 
  |       "    return [1];                              " 
  |       " else                                        " 
  |       "    return [ x, x + y, 2 * v, s + 'world' ]; " 
  |  
  |    symbol_table_t symbol_table; 
  |    expression_t   expression; 
  |    parser_t       parser; 
  |  
  |    symbol_table.add_variable ("x", x); 
  |    symbol_table.add_variable ("y", y); 
  |    symbol_table.add_variable ("z", z); 
  |    symbol_table.add_vector   ("v", v); 
  |    symbol_table.add_stringvar("s", s); 
  |  
  |    parser.compile(expression_str, expression); 
  |  
  |    expression.value(); 
  |  
  |    typedef exprtk::results_context<T> results_context_t; 
  |    const results_context_t& results = expression.results(); 
  |  
  |    if (results.count() == 4) 
  |    { 
  |       T result_x0; 
  |       T result_x1; 
  |       std::string result_s; 
  |       std::vector<T> result_v; 
  |  
  |       results.get_scalar(0, result_x0); 
  |       results.get_scalar(1, result_x1); 
  |       results.get_string(3, result_s ); 
  |       results.get_vector(2, result_v ); 
  |    } 
  |  
  |  
  | It is however recommended that if there is to be only a single flow of 
  | execution  through  the  expression,  that  the  simpler  approach  of 
  | registering external variables of appropriate type be used. 
  |  
  | This method simply requires the variables that are to hold the various 
  | results that are to be computed within the expression to be registered 
  | with an associated symbol_table  instance. Then within the  expression 
  | itself  to  have  the result  variables  be  assigned the  appropriate 
  | values. 
  |  
  |    typedef exprtk::symbol_table<double> symbol_table_t; 
  |    typedef exprtk::expression<double>   expression_t; 
  |    typedef exprtk::parser<double>       parser_t; 
  |  
  |    const std::string expression_string = 
  |       " var x := 123.456;     " 
  |       " var s := 'ijk';       " 
  |       " result0 := x + 78.90; " 
  |       " result1 := s + '123'  " 
  |  
  |    double      result0; 
  |    std::string result1; 
  |  
  |    symbol_table_t symbol_table; 
  |    symbol_table.add_variable ("result0",result0); 
  |    symbol_table.add_stringvar("result1",result1); 
  |  
  |    expression_t expression; 
  |    expression.register_symbol_table(symbol_table); 
  |  
  |    parser_t parser; 
  |    parser.compile(expression_string,expression); 
  |  
  |    expression.value(); 
  |  
  |    printf("Result0: %15.5f\n", result0        ); 
  |    printf("Result1: %s\n"    , result1.c_str()); 
  |  
  |  
  | In the example above, the expression will compute two results. As such 
  | two result variables are defined to hold the values named result0  and 
  | result1 respectively. The first is of scalar type (double), the second 
  | is of  string type.  Once the  expression has  been evaluated, the two 
  | variables will have been updated  with the new result values,  and can 
  | then be further utilised from within the calling host program. 
  |  
  | There will be times when  an expression may have multiple  exit paths, 
  | where not all the paths will be return-statement based. The  following 
  | example builds upon the previous examples, but this time at least  one 
  | path is not return based. 
  |  
  |    typedef exprtk::symbol_table<double> symbol_table_t; 
  |    typedef exprtk::expression<double>   expression_t; 
  |    typedef exprtk::parser<double>       parser_t; 
  |  
  |    double x = 100.0; 
  |    double y = 200.0; 
  |  
  |    symbol_table_t symbol_table; 
  |    expression_t   expression; 
  |    parser_t       parser; 
  |  
  |    symbol_table.add_variable ("x", x); 
  |    symbol_table.add_variable ("y", y); 
  |  
  |    expression.register_symbol_table(symbol_table); 
  |  
  |    const std::string expression_string = 
  |       " for (var i := 0; i < 10; i += 1)       " 
  |       " {                                      " 
  |       "    if (i > x)                          " 
  |       "    {                                   " 
  |       "       return [x + y, 'return-call 1']; " 
  |       "    }                                   " 
  |       "    else if (i > y)                     " 
  |       "    {                                   " 
  |       "       return [x - y, 'return-call 2']; " 
  |       "    }                                   " 
  |       " };                                     " 
  |       "                                        " 
  |       " x / y                                  " 
  |  
  |    parser.compile(expression_str, expression); 
  |  
  |    const auto result = expression.value(); 
  |  
  |    if (expression.return_invoked()) 
  |    { 
  |       const auto results = expression.results(); 
  |  
  |       for (std::size_t i = 0; i <  results.count(); ++i) 
  |       { 
  |          const auto& rtrn_result = results[i]; 
  |          . 
  |          . 
  |          . 
  |       } 
  |    } 
  |    else 
  |    { 
  |       printf("result: %f\n",result); 
  |    } 
  |  
  |  
  | After having called  the value method  on the expression,  calling the 
  | return_invoked method will determine  if the expression completed  due 
  | to a return statement being invoked or if it finished normally. 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 21 - COMPILATION ERRORS] 
  | When attempting to compile  a malformed or otherwise  erroneous ExprTk 
  | expression, the  compilation process  will result  in an  error, as is 
  | indicated  by  the  'compile'  method  returning  a  false  value.   A 
  | diagnostic indicating the first error encountered and its cause can be 
  | obtained by  invoking the  'error' method,  as is  demonstrated in the 
  | following example: 
  |  
  |    if (!parser.compile(expression_string,expression)) 
  |    { 
  |       printf("Error: %s\n", parser.error().c_str()); 
  |       return false; 
  |    } 
  |  
  |  
  | Any error(s) resulting from a failed compilation will be stored in the 
  | parser instance until the next time a compilation is performed. Before 
  | then errors can be enumerated  in the order they occurred  by invoking 
  | the 'get_error' method which itself will return a 'parser_error' type. 
  | A parser_error object will contain an error diagnostic, an error  mode 
  | (or class), and the character position of the error in the  expression 
  | string. The following example demonstrates the enumeration of error(s) 
  | in the event of a failed compilation. 
  |  
  |    typedef exprtk::parser<T>          parser_t; 
  |    typedef exprtk::parser_error::type error_t; 
  |  
  |    if (!parser.compile(expression_string,expression)) 
  |    { 
  |       for (std::size_t i = 0; i < parser.error_count(); ++i) 
  |       { 
  |          typedef exprtk::parser_error::type error_t; 
  |  
  |          error_t error = parser.get_error(i); 
  |  
  |          printf("Error[%02d] Position: %02d Type: [%14s] Msg: %s\n", 
  |                 i, 
  |                 error.token.position, 
  |                 exprtk::parser_error::to_str(error.mode).c_str(), 
  |                 error.diagnostic.c_str()); 
  |       } 
  |  
  |       return false; 
  |    } 
  |  
  |  
  | Assuming the  following expression '2 + (3 / log(1 + x))' which uses a 
  | variable named 'x'  that has not been registered  with the appropriate 
  | symbol_table  instance and  is not  a locally  defined variable,  once 
  | compiled the  above denoted post compilation error handling code shall 
  | produce the following output: 
  |  
  |   Error[00] Pos:17 Type:[Syntax] Msg: ERR184 - Undefined symbol: 'x' 
  |  
  |  
  | For  expressions  comprised  of  multiple  lines,  the  error position 
  | provided in the  parser_error object can  be converted into  a pair of 
  | line and column numbers by invoking the 'update_error' function as  is 
  | demonstrated by the following example: 
  |  
  |    if (!parser.compile(program_str,expression)) 
  |    { 
  |       for (std::size_t i = 0; i < parser.error_count(); ++i) 
  |       { 
  |          typedef exprtk::parser_error::type error_t; 
  |  
  |          error_t error = parser.get_error(i); 
  |  
  |          exprtk::parser_error::update_error(error,program_str); 
  |  
  |          printf("Error[%0lu] at line: %lu column: %lu\n", 
  |                 i, 
  |                 error.line_no, 
  |                 error.column_no); 
  |       } 
  |  
  |       return false; 
  |    } 
  |  
  |  
  | Note32: There are five distinct error modes in ExprTk which denote the 
  | class of an error. These classes are as follows: 
  |  
  |    (a) Syntax 
  |    (b) Token 
  |    (c) Numeric 
  |    (d) Symbol Table 
  |    (e) Lexer 
  |  
  |  
  | (a) Syntax Errors 
  | These are errors  related to invalid  syntax found within  the denoted 
  | expression. Examples are invalid sequences of operators and variables, 
  | incorrect number  of parameters  to functions,  invalid conditional or 
  | loop structures and invalid use of keywords. 
  |  
  |    eg:  'for := sin(x,y,z) + 2 * equal > until[2 - x,3]' 
  |  
  |  
  | (b) Token Errors 
  | Errors in this class relate to  token level errors detected by one  or 
  | more of the following checkers: 
  |  
  |    (1) Bracket Checker 
  |    (2) Numeric Checker 
  |    (3) Sequence Checker 
  |  
  |  
  | (c) Numeric Errors 
  | This class of  error is related  to conversion of  numeric values from 
  | their  string form  to the  underlying numerical  type (float,  double 
  | etc). 
  |  
  | (d) Symbol Table Errors 
  | This is the class of errors related to failures when interacting  with 
  | the registered symbol_table instance. Errors such as not being able to 
  | find,  within  the  symbol_table,  symbols  representing  variables or 
  | functions, to being unable to create new variables in the symbol_table 
  | via the 'unknown symbol resolver' mechanism. 
  |  
  | Note33: The function compositor  also supports  error message handling 
  | similar to how it is  done via the parser. The  following demonstrates 
  | how after a failed function  composition the associated errors can  be 
  | enumerated. 
  |  
  |    typedef exprtk::function_compositor<T>  compositor_t; 
  |    typedef typename compositor_t::function function_t; 
  |  
  |    compositor_t compositor; 
  |  
  |    const bool compositor_result = 
  |    compositor.add( 
  |       function_t("foobar") 
  |       .vars("x","y") 
  |       .expression 
  |       ( " x + y / z " )); 
  |  
  |    if (!compositor_result) 
  |    { 
  |       printf("Error: %s\n", compositor.error().c_str()); 
  |  
  |       for (std::size_t i = 1; i < compositor.error_count(); ++i) 
  |       { 
  |          typedef exprtk::parser_error::type error_t; 
  |  
  |          error_t error = compositor.get_error(i); 
  |  
  |          printf("Err No.: %02d  Pos: %02d  Type: [%14s] Msg: %s\n", 
  |                 static_cast<unsigned int>(i), 
  |                 static_cast<unsigned int>(error.token.position), 
  |                 exprtk::parser_error::to_str(error.mode).c_str(), 
  |                 error.diagnostic.c_str()); 
  |       } 
  |    } 
  |  
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 22 - RUNTIME LIBRARY PACKAGES] 
  | ExprTk includes  a range  of extensions,  that provide functionalities 
  | beyond simple numerical calculations. Currently the available packages 
  | are: 
  |  
  |   +---+--------------------+-----------------------------------+ 
  |   | # |    Package Name    |          Namespace/Type           | 
  |   +---+--------------------+-----------------------------------+ 
  |   | 1 | Basic I/O          | exprtk::rtl::io::package<T>       | 
  |   | 2 | File I/O           | exprtk::rtl::io::file::package<T> | 
  |   | 3 | Vector Operations  | exprtk::rtl::vecops::package<T>   | 
  |   +---+--------------------+-----------------------------------+ 
  |  
  |  
  | In order to make the  features of a specific package  available within 
  | an  expression,  an instance  of  the package  must  be added  to  the 
  | expression's associated  symbol table.  In the  following example, the 
  | file I/O package is made available for the given expression: 
  |  
  |    typedef exprtk::symbol_table<T> symbol_table_t; 
  |    typedef exprtk::expression<T>   expression_t; 
  |    typedef exprtk::parser<T>       parser_t; 
  |  
  |    exprtk::rtl::io::file::package<T> fileio_package; 
  |  
  |    const std::string expression_string = 
  |       " var file_name := 'file.txt';       " 
  |       " var stream    := null;             " 
  |       "                                    " 
  |       " stream := open(file_name,'w');     " 
  |       "                                    " 
  |       " write(stream,'Hello world....\n'); " 
  |       "                                    " 
  |       " close(stream);                     " 
  |       "                                    " 
  |  
  |    symbol_table_t symbol_table; 
  |    symbol_table.add_package(fileio_package); 
  |  
  |    expression_t expression; 
  |    expression.register_symbol_table(symbol_table); 
  |  
  |    parser_t parser; 
  |    parser.compile(expression_string,expression); 
  |  
  |    expression.value(); 
  |  
  |  
  | (1) Basic I/O functions: 
  |  
  |    (a) print 
  |    (b) println 
  |  
  | (2) File I/O functions: 
  |  
  |    (a) open         (b) close 
  |    (c) write        (d) read 
  |    (e) getline      (f) eof 
  |  
  | (3) Vector Operations functions: 
  |  
  |    (a) all_true     (b) all_false 
  |    (c) any_true     (d) any_false 
  |    (e) assign       (f) count 
  |    (g) copy         (h) reverse 
  |    (i) rotate-left  (j) rotate-right 
  |    (k) shift-left   (l) shift-right 
  |    (m) sort         (n) nth_element 
  |    (o) iota         (p) sumk 
  |    (q) axpy         (r) axpby 
  |    (s) axpyz        (t) axpbyz 
  |    (u) axpbz        (v) dot 
  |    (w) dotk         (x) diff 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 23 - HELPERS & UTILS] 
  | The  ExprTk library  provides a  series of  usage simplifications  via 
  | helper routines that combine various processes into a single 'function 
  | call'  making  certain  actions   easier  to  carry  out   though  not 
  | necessarily in the most efficient way possible. A list of the routines 
  | are as follows: 
  |  
  |    (a) collect_variables 
  |    (b) collect_functions 
  |    (c) compute 
  |    (d) integrate 
  |    (e) derivative 
  |    (f) second_derivative 
  |    (g) third_derivative 
  |  
  |  
  | (a) collect_variables 
  | This function will collect all the variable symbols in a given  string 
  | representation of an expression and  return them in an STL  compatible 
  | sequence  data structure  (eg: std::vector,  dequeue etc)  specialised 
  | upon a std::string type. If an error occurs during the parsing of  the 
  | expression  then  the return  value  of the  function  will be  false, 
  | otherwise it will be  true. An example use  of the given routine is as 
  | follows: 
  |  
  |    const std::string expression = "x + abs(y / z)" 
  |  
  |    std::vector<std::string> variable_list; 
  |  
  |    if (exprtk::collect_variables(expression, variable_list)) 
  |    { 
  |       for (const auto& var : variable_list) 
  |       { 
  |          ... 
  |       } 
  |    } 
  |    else 
  |      printf("An error occurred."); 
  |  
  |  
  | (b) collect_functions 
  | This function will collect all the function symbols in a given  string 
  | representation of an expression and  return them in an STL  compatible 
  | sequence  data structure  (eg: std::vector,  dequeue etc)  specialised 
  | upon a std::string type. If an error occurs during the parsing of  the 
  | expression  then  the return  value  of the  function  will be  false, 
  | otherwise it  will be true. An example  use of the given routine is as 
  | follows: 
  |  
  |    const std::string expression = "x + abs(y / cos(1 + z))" 
  |  
  |    std::deque<std::string> function_list; 
  |  
  |    if (exprtk::collect_functions(expression, function_list)) 
  |    { 
  |       for (const auto& func : function_list) 
  |       { 
  |          ... 
  |       } 
  |    } 
  |    else 
  |      printf("An error occurred."); 
  |  
  |  
  | Note34: When  either the  'collect_variables'  or  'collect_functions' 
  | free functions return  true - that  does not necessarily  indicate the 
  | expression itself is  valid. It is  still possible that  when compiled 
  | the expression may have certain  'type' related errors - though  it is 
  | highly likely  that no  semantic errors  will occur  if either  return 
  | true. 
  |  
  | Note35: The default interface provided for both the  collect_variables 
  | and collect_functions  free_functions, assumes  that expressions  will 
  | only be  utilising the  ExprTk reserved  functions (eg:  abs, cos, min 
  | etc). When user defined functions are  to be used in an expression,  a 
  | symbol_table  instance  containing  said functions  can  be  passed to 
  | either routine, and  will be incorporated  during the compilation  and 
  | Dependent Entity  Collection processes.  In the  following example,  a 
  | user  defined  free  function   named  'foo'  is  registered   with  a 
  | symbol_table.  Finally  the   symbol_table  instance  and   associated 
  | expression string are passed to the exprtk::collect_functions routine. 
  |  
  |    template <typename T> 
  |    T foo(T v) 
  |    { 
  |       return std::abs(v + T(2)) / T(3); 
  |    } 
  |  
  |    ...... 
  |  
  |    exprtk::symbol_table<T> sym_tab; 
  |  
  |    symbol_table.add_function("foo",foo); 
  |  
  |    const std::string expression = "x + foo(y / cos(1 + z))" 
  |  
  |    std::deque<std::string> function_list; 
  |  
  |    if (exprtk::collect_functions(expression, sym_tab, function_list)) 
  |    { 
  |       for (const auto& func : function_list) 
  |       { 
  |          ... 
  |       } 
  |    } 
  |    else 
  |      printf("An error occurred."); 
  |  
  |  
  | (c) compute 
  | This free function  will compute the  value of an  expression from its 
  | string form.  If an  invalid expression  is passed,  the result of the 
  | function will be false indicating an error, otherwise the return value 
  | will  be  true  indicating success.  The  compute  function has  three 
  | overloads, the definitions of which are: 
  |  
  |    (1) No variables 
  |    (2) One variable called x 
  |    (3) Two variables called x and y 
  |    (3) Three variables called x, y and z 
  |  
  |  
  | Example uses of  each of the  three overloads for  the compute routine 
  | are as follows: 
  |  
  |    T result = T(0); 
  |  
  |    // No variables overload 
  |    const std::string no_vars = "abs(1 - (3 / pi)) * 5" 
  |  
  |    if (!exprtk::compute(no_vars,result)) 
  |       printf("Failed to compute: %s",no_vars.c_str()); 
  |    else 
  |       printf("Result: %15.5f\n",result); 
  |  
  |    // One variable 'x' overload 
  |    T x = T(123.456); 
  |  
  |    const std::string one_var = "abs(x - (3 / pi)) * 5" 
  |  
  |    if (!exprtk::compute(one_var, x, result)) 
  |       printf("Failed to compute: %s",one_var.c_str()); 
  |    else 
  |       printf("Result: %15.5f\n",result); 
  |  
  |    // Two variables 'x' and 'y' overload 
  |    T y = T(789.012); 
  |  
  |    const std::string two_var = "abs(x - (y / pi)) * 5" 
  |  
  |    if (!exprtk::compute(two_var, x, y, result)) 
  |       printf("Failed to compute: %s",two_var.c_str()); 
  |    else 
  |       printf("Result: %15.5f\n",result); 
  |  
  |    // Three variables 'x', 'y' and 'z' overload 
  |    T z = T(345.678); 
  |  
  |    const std::string three_var = "abs(x - (y / pi)) * z" 
  |  
  |    if (!exprtk::compute(three_var, x, y, z, result)) 
  |       printf("Failed to compute: %s",three_var.c_str()); 
  |    else 
  |       printf("Result: %15.5f\n",result); 
  |  
  |  
  | (d) integrate 
  | This free function will attempt to perform a numerical integration  of 
  | a single variable compiled expression over a specified range and  step 
  | size. The numerical  integration is based  on the three  point form of 
  | Simpson's rule. The  integrate function has  two overloads, where  the 
  | variable of integration can  either be passed as  a reference or as  a 
  | name in string form. Example usage of the function is as follows: 
  |  
  |    typedef exprtk::symbol_table<T> symbol_table_t; 
  |    typedef exprtk::expression<T>   expression_t; 
  |    typedef exprtk::parser<T>       parser_t; 
  |  
  |    const std::string expression_string = "sqrt(1 - (x^2))" 
  |  
  |    T x = T(0); 
  |  
  |    symbol_table_t symbol_table; 
  |    symbol_table.add_variable("x",x); 
  |  
  |    expression_t expression; 
  |    expression.register_symbol_table(symbol_table); 
  |  
  |    parser_t parser; 
  |    parser.compile(expression_string,expression); 
  |  
  |    .... 
  |  
  |    // Integrate in domain [-1,1] using a reference to x variable 
  |    T area1 = exprtk::integrate(expression, x, T(-1), T(1)); 
  |  
  |    // Integrate in domain [-1,1] using name of x variable 
  |    T area2 = exprtk::integrate(expression, "x", T(-1), T(1)); 
  |  
  |  
  | (e) derivative 
  | This free function will attempt to perform a numerical differentiation 
  | of a single variable compiled expression at a given point for a  given 
  | epsilon, using a  variant of Newton's  difference quotient called  the 
  | five-point stencil method. The derivative function has two  overloads, 
  | where  the  variable of  differentiation  can either  be  passed as  a 
  | reference or as a name in string form. Example usage of the derivative 
  | function is as follows: 
  |  
  |    typedef exprtk::symbol_table<T> symbol_table_t; 
  |    typedef exprtk::expression<T>   expression_t; 
  |    typedef exprtk::parser<T>       parser_t; 
  |  
  |    const std::string expression_string = "sqrt(1 - (x^2))" 
  |  
  |    T x = T(0); 
  |  
  |    symbol_table_t symbol_table; 
  |    symbol_table.add_variable("x",x); 
  |  
  |    expression_t expression; 
  |    expression.register_symbol_table(symbol_table); 
  |  
  |    parser_t parser; 
  |    parser.compile(expression_string,expression); 
  |  
  |    .... 
  |  
  |    // Differentiate expression at value of x = 12.3 using a reference 
  |    // to the x variable 
  |    x = T(12.3); 
  |    T derivative1 = exprtk::derivative(expression, x); 
  |  
  |    // Differentiate expression where value x = 45.6 using name 
  |    // of the x variable 
  |    x = T(45.6); 
  |    T derivative2 = exprtk::derivative(expression, "x"); 
  |  
  |  
  | (f) second_derivative 
  | This  free  function  will  attempt  to  perform  a  numerical  second 
  | derivative of a single variable  compiled expression at a given  point 
  | for a given epsilon, using  a variant of Newton's difference  quotient 
  | method. The second_derivative function  has  two overloads, where  the 
  | variable of differentiation can either be passed as a reference or  as 
  | a name in string form. Example usage of the second_derivative function 
  | is as follows: 
  |  
  |    typedef exprtk::symbol_table<T> symbol_table_t; 
  |    typedef exprtk::expression<T>   expression_t; 
  |    typedef exprtk::parser<T>       parser_t; 
  |  
  |    const std::string expression_string = "sqrt(1 - (x^2))" 
  |  
  |    T x = T(0); 
  |  
  |    symbol_table_t symbol_table; 
  |    symbol_table.add_variable("x",x); 
  |  
  |    expression_t expression; 
  |    expression.register_symbol_table(symbol_table); 
  |  
  |    parser_t parser; 
  |    parser.compile(expression_string,expression); 
  |  
  |    .... 
  |  
  |    // Second derivative of expression where value of x = 12.3 using a 
  |    // reference to x variable 
  |    x = T(12.3); 
  |    T derivative1 = exprtk::second_derivative(expression,x); 
  |  
  |    // Second derivative of expression where value of x = 45.6 using 
  |    // name of x variable 
  |    x = T(45.6); 
  |    T derivative2 = exprtk::second_derivative(expression, "x"); 
  |  
  |  
  | (g) third_derivative 
  | This  free  function  will  attempt  to  perform  a  numerical   third 
  | derivative of a single variable  compiled expression at a given  point 
  | for a given epsilon, using  a variant of Newton's difference  quotient 
  | method. The  third_derivative function  has two  overloads, where  the 
  | variable of differentiation can either be passed as a reference or  as 
  | a name in string form. Example  usage of the third_derivative function 
  | is as follows: 
  |  
  |    typedef exprtk::symbol_table<T> symbol_table_t; 
  |    typedef exprtk::expression<T>   expression_t; 
  |    typedef exprtk::parser<T>       parser_t; 
  |  
  |    const std::string expression_string = "sqrt(1 - (x^2))" 
  |  
  |    T x = T(0); 
  |  
  |    symbol_table_t symbol_table; 
  |    symbol_table.add_variable("x",x); 
  |  
  |    expression_t expression; 
  |    expression.register_symbol_table(symbol_table); 
  |  
  |    parser_t parser; 
  |    parser.compile(expression_string,expression); 
  |  
  |    .... 
  |  
  |    // Third derivative of expression where value of x = 12.3 using a 
  |    // reference to the x variable 
  |    x = T(12.3); 
  |    T derivative1 = exprtk::third_derivative(expression, x); 
  |  
  |    // Third derivative of expression where value of x = 45.6 using 
  |    // name of the x variable 
  |    x = T(45.6); 
  |    T derivative2 = exprtk::third_derivative(expression, "x"); 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 24 - RUNTIME CHECKS] 
  | The  ExprTk library   provides the ability  to perform runtime  checks 
  | during expression evaluation so as to ensure memory access  violations 
  | errors  are  caught and  handled  without causing further  issues. The 
  | checks typically cover: 
  |  
  |    1. Vector access and handling 
  |    2. String access and handling 
  |    3. Loop iteration checks 
  |    4. Compilation checkpointing 
  |    5. Assert statements 
  |  
  |  
  | (1) Vector Access Runtime Checks 
  | Expressions that contain vectors where elements of the vectors may  be 
  | accessed using  indexes that  can only  be determined  at runtime  may 
  | result  in  memory  access violations  when the  index is  out of  the 
  | vector's  bound.  Some  examples  of  problematic  expressions  are as 
  | follows: 
  |  
  |    1. vec[i] 
  |    2. vec[i + j] 
  |    3. vec[i + 10] 
  |    4. vec[i + vec[]] := x + y 
  |    5. vec[i + j] <=> vec[i] 
  |    6. vec[i + j] := (vec1 + vec2)[i + j] 
  |  
  |  
  | In the above expressions,  it is assumed that  the values used in  the 
  | index operator  may either  exceed the  vector bounds  or precede  the 
  | vector's start, In  short, the indexes  may not necessarily  be within 
  | the range [0,vec[]). 
  |  
  | ExprTk provides the ability to inject a runtime check at the point  of 
  | index evaluation and  handle situations where  the index violates  the 
  | vector's  bounds.  This  capability is  done  by  registering a  user- 
  | implemented Vector Access Runtime  Check (VARTC) to the  parser before 
  | expression compilation. Initially a VARTC can be defined as follows: 
  |  
  |    struct my_vector_access_rtc final : 
  |       public exprtk::vector_access_runtime_check 
  |    { 
  |       bool handle_runtime_violation(violation_context& context) 
  |       override 
  |       { 
  |          // Handling of the violation 
  |          return ...; 
  |       } 
  |    }; 
  |  
  |  
  | Then an instance of the VARTC can be registered with a parser instance 
  | as follows: 
  |  
  |    my_vector_access_rtc vartc; 
  |  
  |    exprtk::symbol_table<T> symbol_table; 
  |  
  |    T i; 
  |    T x; 
  |    T y; 
  |    std::vector<T> vec = { 0, 1, 2, 3, 4 }; 
  |  
  |    symbol_table.add_variable("i"  , i  ); 
  |    symbol_table.add_variable("x"  , x  ); 
  |    symbol_table.add_variable("y"  , y  ); 
  |    symbol_table.add_vector  ("vec", vec); 
  |  
  |    exprtk::expression<T> expression; 
  |    exprtk::parser<T> parser; 
  |  
  |    parser.register_vector_access_runtime_check(vartc); 
  |  
  |    std::string expression = "vec[i + vec[]] := x + y" 
  |  
  |    parser.compile(expression_str, expression); 
  |  
  |    try 
  |    { 
  |       expression.value(); 
  |    } 
  |    catch (std::runtime_error& rte) 
  |    { 
  |       printf("Exception: %s\n", rte.what()); 
  |    } 
  |  
  |  
  | Note36: The  lifetime of  any parser  or expression  instance must not 
  | exceed that of any VARTC instance that has been registered with it. 
  |  
  | When a vector access violation occurs, the registered VARTC instance's 
  | handle_runtime_violation method  will be  invoked, coupled  with it  a 
  | violation_context shall  be provided  that will  contain the following 
  | members: 
  |  
  |   1. base_ptr: Of type void*,  which points to the  first element 
  |      of the vector.  The base_ptr can  also be used  as a key  to 
  |      determine the  vector upon  which the  access violation  has 
  |      occurred. 
  |  
  |   2. end_ptr : Of type void*, which points to one position  after 
  |      the last element of the vector 
  |  
  |   3. access_ptr: Of  type void*,  points to  the memory  location 
  |      which is the base_ptr offset by the derived index value. 
  |  
  |   4. type_size: Size of the vector's element type in bytes. This 
  |      value can be used to  determine  the number of  elements in 
  |      the vector based on the base_ptr and end_ptr. 
  |  
  |  
  | The implementation of the handle_runtime_violation method can at this 
  | point perform various actions such as: 
  |  
  |   1. Log the violation 
  |   2. Throw an exception (eg: std::runtime_error) 
  |   3. Remedy the access_ptr to allow for the evaluation to continue 
  |  
  |  
  | Note37: When employing option [3],  handle_runtime_violation needs  to 
  | return  true,  otherwise  the  caller  will assume an unhandled access 
  | violation and default to using the base_ptr. 
  |  
  | It is  recommended, at  the  very least,  to throw an  exception  when 
  | handling vector access violations and to only consider option [3] when 
  | the the ramifications of changing the access_ptr are well understood. 
  |  
  | The following are simple examples of how the  handle_runtime_violation 
  | can be implemented. 
  |  
  | Example 1: Log the access violation to stdout and then throw a runtime 
  | error exception: 
  |  
  |    bool handle_runtime_violation(violation_context& context) override 
  |    { 
  |       printf("ERROR -  Runtime vector access violation. " 
  |              "base: %p end: %p access: %p typesize: %lu\n", 
  |              context.base_ptr  , 
  |              context.end_ptr   , 
  |              context.access_ptr, 
  |              context.type_size); 
  |  
  |       throw std::runtime_error("Runtime vector access violation."); 
  |       return false; 
  |    } 
  |  
  |  
  | Example 2: Handle the access violation by resetting the access pointer 
  | to the last value in the vector. 
  |  
  |    bool handle_runtime_violation(violation_context& context) override 
  |    { 
  |       context.access_ptr = 
  |          static_cast<char*>(context.end_ptr) - context.type_size; 
  |       return true; 
  |    } 
  |  
  |  
  | Note38: The return value  of true  in the above handler method signals 
  | the caller to continue the vector access using the updated access_ptr. 
  |  
  |  
  | (2) String Access Runtime Checks 
  | Expressions that contain strings  where elements or substrings  of the 
  | strings may be accessed using  indexes that can only be  determined at 
  | runtime may result in memory access violations when the index or range 
  | is out of the string's bound. Examples of problematic expressions  are 
  | as follows: 
  |  
  |    1. s[i : j + k] 
  |    2. s[i : j + k][-x : y] 
  |    3. (s1 + s2)[i : j + k] 
  |    4. '01234'[5 + i] 
  |    5. s += s[i : j + k] 
  |    6. s[i : j + k] := 'chappy days'[1 : ] 
  |  
  |  
  | To enable string access runtime checks all  one needs to  do is simply 
  | use the following  define before the  ExprTk header is  included or as 
  | part of the compilation define parameters: 
  |  
  |    exprtk_enable_range_runtime_checks 
  |  
  |  
  | When  the above  define is used,  and a string  related runtime access 
  | violation occurs  a std::runtime_error  exception will  be thrown. The 
  | following  demonstrates  the  general  flow  of  handling  the  access 
  | violation: 
  |  
  |    parser.compile(expression_string, expression) 
  |    . 
  |    . 
  |    try 
  |    { 
  |       expression.value(); 
  |    } 
  |    catch (std::runtime_error& rte) 
  |    { 
  |       printf("Exception: %s\n", rte.what()); 
  |    } 
  |  
  |  
  | (3) Loop Iteration Checks 
  | Expressions that contain loop structures (eg: for/while/repeat et  al) 
  | can be problematic from a usage point of view due to the difficulty in 
  | determining the following: 
  |  
  |    1. Will the loop ever complete (aka is this an infinite loop?) 
  |    2. Maximum loop execution time 
  |  
  |  
  | ExprTk provides  the ability  to inject  a runtime  check within  loop 
  | conditionals, and to  have the result  of the check  either signal the 
  | loop to continue or for the check to raise a loop violation error. 
  |  
  | The process involves instantiating a user defined   loop_runtime_check 
  | (LRTC), registering  the instance  with a  exprtk::parser instance and 
  | specifying  which  loop types  the  check is  to  performed upon.  The 
  | following code demonstrates a how custom LRTC can be instantiated  and 
  | registered with the associated parser: 
  |  
  |    typedef exprtk::parser<T> parser_t; 
  |    typedef exprtk::loop_runtime_check loop_runtime_check_t; 
  |  
  |    my_loop_rtc loop_rtc; 
  |    loop_runtime_check.loop_set = loop_runtime_check_t::e_all_loops; 
  |    loop_runtime_check.max_loop_iterations = 100000; 
  |  
  |    parser_t parser; 
  |  
  |    parser.register_loop_runtime_check(loop_rtc); 
  |  
  |  
  | The following is an example of how one could derive from and implement 
  | a custom loop_runtime_check: 
  |  
  |    struct my_loop_rtc final : exprtk::loop_runtime_check 
  |    { 
  |  
  |       bool check() override 
  |       { 
  |          // 
  |          return ... 
  |       } 
  |  
  |       void handle_runtime_violation 
  |          (const exprtk::violation_context&) override 
  |       { 
  |          throw std::runtime_error("Loop runtime violation."); 
  |       } 
  |    }; 
  |  
  |  
  | In the above  code, if either  the check method  returns false or  the 
  | loop  iteration  count  exceeds  the  max_loop_iterations  value,  the 
  | handle_runtime_violation method  will be  invoked, coupled  with it  a 
  | violation_context shall  be provided  that will  contain the following 
  | members: 
  |  
  |    1. loop: Of  type loop_types.  This value  denotes the  type of 
  |       loop that triggered the violation (e_for_loop, e_while_loop, 
  |       e_repeat_until_loop). 
  |  
  |    2. violation:  Of type  type. This  value denotes  the type  of 
  |       violation (e_iteration_count, e_timeout) 
  |  
  |    3. iteration_count: Of type uint64_t. The number of  iterations 
  |       that the triggering loop has executed since the start of the 
  |       expression. 
  |  
  |  
  | Note39: The  lifetime of  any parser  or expression  instance must not 
  | exceed that of any LRTC instance that has been registered with it. 
  |  
  | The following is an  example implementation of an  LRTC that 
  | supports loop timeout violations: 
  |  
  |    struct timeout_loop_rtc final : exprtk::loop_runtime_check 
  |    { 
  |       using time_point_t = 
  |          std::chrono::time_point<std::chrono::steady_clock>; 
  |  
  |       std::size_t iterations_ = 0; 
  |       time_point_t timeout_tp_; 
  |  
  |       bool check() override 
  |       { 
  |          if (std::chrono::steady_clock::now() >= timeout_tp_) 
  |          { 
  |             // handle_runtime_violation shall be invoked 
  |             return false; 
  |          } 
  |  
  |          return true; 
  |       } 
  |  
  |       void handle_runtime_violation 
  |          (const exprtk::violation_context&) override 
  |       { 
  |          throw std::runtime_error("Loop timed out"); 
  |       } 
  |  
  |       void set_timeout_time(const time_point_t& timeout_tp) 
  |       { 
  |          timeout_tp_ = timeout_tp; 
  |       } 
  |    }; 
  |  
  |  
  | In the above code, the check method shall be invoked on each iteration 
  | of the associated loop. Within the method the current time is compared 
  | to the setup timeout time-point, in the event the current time exceeds 
  | the timeout, the method returns false, triggering the violation, which 
  | in turn will result in the handle_runtime_violation being invoked. 
  |  
  | The following code demonstrates how the above defined LRTC can be used 
  | to ensure that at the very least the loop portion(s) of an  expression 
  | will never exceed a given amount of execution time. 
  |  
  |    typedef exprtk::parser<T> parser_t; 
  |    typedef exprtk::loop_runtime_check loop_runtime_check_t; 
  |  
  |    my_loop_rtc loop_rtc; 
  |    loop_rtc.loop_set = loop_runtime_check_t::e_all_loops; 
  |    loop_rtc.max_loop_iterations = 100000; 
  |  
  |    parser_t parser; 
  |  
  |    parser.register_loop_runtime_check(loop_rtc); 
  |    . 
  |    . 
  |    . 
  |    . 
  |    using std::chrono; 
  |    const auto max_duration = seconds(25); 
  |  
  |    try 
  |    { 
  |       loop_rtc.set_timeout_time(steady_clock::now() + max_duration); 
  |       expression.value(); 
  |  
  |       loop_rtc.set_timeout_time(steady_clock::now() + max_duration); 
  |       expression.value(); 
  |  
  |       loop_rtc.set_timeout_time(steady_clock::now() + max_duration); 
  |       expression.value(); 
  |  
  |    } 
  |    catch(std::runtime_error& exception) 
  |    { 
  |       printf("Exception: %s\n",exception.what()); 
  |    } 
  |  
  |  
  | (4) Compilation Process Checkpointing 
  | When compiling an expression, one may require the compilation  process 
  | to periodically  checkpoint its  internal state,  subsequently at  the 
  | checkpoint one can then make the decision to continue the  compilation 
  | process or to immediately terminate and return. 
  |  
  | The following are reasons one  may want to checkpoint the  compilation 
  | process: 
  |  
  |    1. Determine if the compilation process has run for far too long 
  |    2. Determine if the current stack frame size exceeds a limit 
  |    3. Enforce an external termination request 
  |  
  |  
  | ExprTk  provides  the  ability   to  inject  a  checkpoint   into  the 
  | compilation  process  that   will  be  evaluated   periodically.  This 
  | capability is achieved  by registering a  user-implemented compilation 
  | check (CCK) to the parser  before expression  compilation. Initially a 
  | CCK can be defined as follows: 
  |  
  |    struct compilation_timeout_check final : 
  |       public exprtk::compilation_check 
  |    { 
  |       bool continue_compilation(compilation_context& context) 
  |       override 
  |       { 
  |          // Determine if compilation should continue 
  |          return ...; 
  |       } 
  |    }; 
  |  
  |  
  | An  example checkpoint  use-case could  be that  we do  not want  the 
  | compilation process to take longer than a maximum defined period,  eg: 
  | five seconds. The associated compilation check implementation could be 
  | as follows: 
  |  
  |    struct my_compilation_timeout_check final : 
  |       public exprtk::compilation_check 
  |    { 
  |  
  |       bool continue_compilation(compilation_context& context) 
  |       override 
  |       { 
  |          static constexpr std::size_t max_iters_per_check = 1000; 
  |  
  |          if (++iterations_ >= max_iters_per_check) 
  |          { 
  |             if (std::chrono::steady_clock::now() >= timeout_tp_) 
  |             { 
  |                context.error_message = "Compilation has timed-out" 
  |                return false; 
  |             } 
  |  
  |             iterations_ = 0; 
  |          } 
  |  
  |          return true; 
  |       } 
  |  
  |       using time_point_t = std::chrono::time_point<std::chrono::steady_clock>; 
  |  
  |       void set_timeout_time(const time_point_t& timeout_tp) 
  |       { 
  |          timeout_tp_ = timeout_tp; 
  |       } 
  |  
  |       std::size_t iterations_ = 0; 
  |       time_point_t timeout_tp_; 
  |    }; 
  |  
  |  
  | Usage of the above defined compilation check will require  registering 
  | the  check  with the  parser,  setting up  the  expiry time  and  then 
  | proceeding  to  compile the  expression.  The following  is  a general 
  | outline of what will be needed: 
  |  
  |    typedef exprtk::expression<T> expression_t; 
  |    typedef exprtk::parser<T>     parser_t; 
  |  
  |    expression_t expression; 
  |  
  |    my_compilation_timeout_check compilation_timeout_check; 
  |  
  |    parser_t parser; 
  |    parser. 
  |       register_compilation_timeout_check(compilation_timeout_check); 
  |  
  |    const auto max_duration = std::chrono::seconds(5); 
  |    const auto timeout_tp   = 
  |       std::chrono::steady_clock::now() + max_duration; 
  |  
  |    compilation_timeout_check.set_timeout_time(timeout_tp); 
  |  
  |    if (!parser.compile(large_expression_string, expression)) 
  |    { 
  |       printf("Error: %s\t\n", parser.error().c_str()); 
  |       return; 
  |    } 
  |  
  |  
  | (5) Assert statements 
  | ExprTk supports the  use of assert  statements to verify  pre and post 
  | conditions during the evaluation of expressions. The assert statements 
  | are only active when a user defined assert handler is registered  with 
  | the parser before expression compilation, otherwise they are  compiled 
  | out,  this is  similar to  how asserts  are included/excluded  in C++ 
  | coupled with  the definition  of NDEBUG.  The assert  syntax has three 
  | variations as described below: 
  |  
  |    assert(x + y > i); 
  |    assert(x + y > i, 'assert statement 1'); 
  |    assert(x + y > i, 'assert statement 1', 'ASSERT01'); 
  |  
  |  
  | The three assert statement input parameters are as follows: 
  |  
  |    1. assert condition (mandatory) 
  |    2. assert message   (optional) 
  |    3. assert id        (optional) 
  |  
  |  
  | The  assert  condition  is essentially  a  boolean  statement that  is 
  | expected to  be true  during evaluation.  The other  two parameters of 
  | assert message and ID are  string values that are intended  to provide 
  | feedback  to  the  handler  and  to  ensure the  uniqueness  of assert 
  | statement respectively.  The three  parameters denoted  above and  the 
  | offset of the  assert statement from  the beginning of  the expression 
  | are  placed  inside  assert_context that  is provided  as part  of the 
  | assert_check  handler.  A  user defined  assert_check  handler  can be 
  | defined as follows: 
  |  
  |    struct my_assert_handler final : public exprtk::assert_check 
  |    { 
  |       void handle_assert(const assert_context& ctxt) override 
  |       { 
  |          printf("condition: [%s] \n", ctxt.condition.c_str()); 
  |          printf("message:   [%s] \n", ctxt.message  .c_str()); 
  |          printf("id:        [%s] \n", ctxt.id       .c_str()); 
  |          printf("offset:    [%lu]\n", ctxt.offet            ); 
  |          // throw std::runtime_error(.....); 
  |       } 
  |    }; 
  |  
  |  
  | Once the  assert_check handler  has been  registered with  the parser, 
  | expressions that  contain assert  statements will  have their  asserts 
  | compiled in as part final evaluable expression instance: 
  |  
  |    typedef exprtk::symbol_table<T> symbol_table_t; 
  |    typedef exprtk::expression<T>   expression_t; 
  |    typedef exprtk::parser<T>       parser_t; 
  |  
  |    const std::string program = 
  |       " var x := 4;                             " 
  |       "                                         " 
  |       " for (var i := 0; i < 10; i += 1)        " 
  |       " {                                       " 
  |       "    assert(i < x, 'assert statement 1'); " 
  |       " }                                       " 
  |  
  |    my_assert_handler handler; 
  |  
  |    expression_t expression; 
  |    parser_t parser; 
  |  
  |    parser.register_assert_check(handler); 
  |    parser.compile(program, expression); 
  |  
  |  
  | (6) Runtime Check Overheads 
  | All of the above mentioned runtime checks will incur an execution time 
  | overhead during the evaluation of expressions. This is an  unfortunate 
  | but necessary  side-effect of  the process  when runtime  safety is of 
  | concern. 
  |  
  | A recommendation to consider, that is not demonstrated above, is  that 
  | in the check method of the  LRTC, one should not evaluate the  timeout 
  | condition  on  every call  to  check (aka  on  every loop  iteration). 
  | Instead a counter should be maintained  and incremented on each   call 
  | and  when  the  counter  exceeds  some  predefined  amount  (eg: 10000 
  | iterations),  then  the  timeout based  check  can  be preformed.  The 
  | reasoning here  is that  incrementing an  integer should  be far  less 
  | expensive than computing the current "now" time-point. 
  |  
  |  
  | (7) Runtime Check Limitations 
  | The available  RTC mechanisms  in ExprTk  are limited  to implementing 
  | said checks only within ExprTk based syntax sections of an expression. 
  | The  RTCs  will  not  be  active  within  user  defined  functions, or 
  | composited functions  that have  been compiled  with parser  instances 
  | that don't have the same set of RTC configurations enabled. 
  |  
  |  
  | (8) Runtime Handlers 
  | When implementing stateful run-time check handlers one must be careful 
  | to ensure the handler is setup correctly or reset between calls to the 
  | expression::value or parser::compile methods. 
  |  
  | The following example  code utilises the  compilation timeout RTC  and 
  | expression loop duration  RTC examples from  above to demonstrate  the 
  | need  to  reset the  internal  state of  the  various handlers  before 
  | compilation and valuation processes are invoked, as not doing so  will 
  | affect the ability for  the next expression in  the list to either  be 
  | correctly  compiled or  evaluated due  to the  potential of  erroneous 
  | timeouts occurring. 
  |  
  |    typedef exprtk::expression<T> expression_t; 
  |    typedef exprtk::parser<T>     parser_t; 
  |  
  |    my_compilation_timeout_check compilation_timeout_check; 
  |  
  |    my_loop_rtc loop_rtc; 
  |    loop_rtc.loop_set = loop_runtime_check_t::e_all_loops; 
  |    loop_rtc.max_loop_iterations = 100000; 
  |  
  |    parser_t parser; 
  |    parser.register_loop_runtime_check(loop_rtc); 
  |    parser. 
  |       register_compilation_timeout_check(compilation_timeout_check); 
  |  
  |    const auto compile_timeout_tp = []() 
  |    { 
  |       const auto max_duration = std::chrono::seconds(5); 
  |       return std::chrono::steady_clock::now() + max_duration; 
  |    }; 
  |  
  |    const auto loop_timeout_tp = []() 
  |    { 
  |       const auto max_duration = std::chrono::seconds(10); 
  |       return std::chrono::steady_clock::now() + max_duration; 
  |    }; 
  |  
  |    const std::vector<std::string> expressions = 
  |    { 
  |       "x + y / 2", 
  |       "sin(x) / cos(y) + 1", 
  |       "clamp(-1, sin(2 * pi * x) + cos(y / 2 * pi), +1)" 
  |    }; 
  |  
  |    for (const auto& expr_str : expressions) 
  |    { 
  |       // Reset the timeout for the compilation RTC 
  |       compilation_timeout_check 
  |          .set_timeout_time(compile_timeout_tp()); 
  |  
  |       expression_t expression; 
  |  
  |       if (!parser.compile(large_expression_string, expression)) 
  |       { 
  |          printf("Error: %s\t\n", parser.error().c_str()); 
  |          continue; 
  |       } 
  |  
  |       try 
  |       { 
  |          // Reset the timeout for the loop duration RTC 
  |          loop_rtc.set_timeout_time(loop_timeout_tp()); 
  |  
  |          expression.value(); 
  |       } 
  |       catch(std::runtime_error& exception) 
  |       { 
  |          printf("Exception: %s\n Expression: %s\n", 
  |                 exception.what(), 
  |                 expr_str.c_str()); 
  |       } 
  |    } 
  |  
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 25 - BENCHMARKING] 
  | As part of the ExprTk package there is an expression benchmark utility 
  | named 'exprtk_benchmark'. The utility attempts to determine expression 
  | evaluation  speed (or  rate of  evaluations -  evals  per  second), by 
  | evaluating each expression numerous times and mutating the  underlying 
  | variables  of  the  expression between  each  evaluation.  The utility 
  | assumes any  valid ExprTk  expression (containing  conditionals, loops 
  | etc), however  it will  only make  use of  a predefined  set of scalar 
  | variables, namely: a, b, c, x, y, z and w. That being said expressions 
  | themselves  can  contain any  number  of local  variables,  vectors or 
  | strings. There are two modes of operation: 
  |  
  |    (1) Default 
  |    (2) User Specified Expressions 
  |  
  |  
  | (1) Default 
  | The default mode is  enabled simply by executing  the exprtk_benchmark 
  | binary with no command line parameters. In this mode a predefined  set 
  | of expressions will be evaluated in three phases: 
  |  
  |    (a) ExprTk evaluation 
  |    (b) Native evaluation 
  |    (c) ExprTk parse 
  |  
  |  
  | In the first two  phases (a and b)  a list of predefined  (hard-coded) 
  | expressions  will  be  evaluated using  both  ExprTk  and native  mode 
  | implementations.  This  is  done so  as  to  compare evaluation  times 
  | between ExprTk and native implementations. The set of expressions used 
  | are as follows: 
  |  
  |    (01) (y + x) 
  |    (02) 2 * (y + x) 
  |    (03) (2 * y + 2 * x) 
  |    (04) ((1.23 * x^2) / y) - 123.123 
  |    (05) (y + x / y) * (x - y / x) 
  |    (06) x / ((x + y) + (x - y)) / y 
  |    (07) 1 - ((x * y) + (y / x)) - 3 
  |    (08) (5.5 + x) + (2 * x - 2 / 3 * y) * (x / 3 + y / 4) + (y + 7.7) 
  |    (09) 1.1x^1 + 2.2y^2 - 3.3x^3 + 4.4y^15 - 5.5x^23 + 6.6y^55 
  |    (10) sin(2 * x) + cos(pi / y) 
  |    (11) 1 - sin(2 * x) + cos(pi / y) 
  |    (12) sqrt(111.111 - sin(2 * x) + cos(pi / y) / 333.333) 
  |    (13) (x^2 / sin(2 * pi / y)) - x / 2 
  |    (14) x + (cos(y - sin(2 / x * pi)) - sin(x - cos(2 * y / pi))) - y 
  |    (15) clamp(-1.0, sin(2 * pi * x) + cos(y / 2 * pi), +1.0) 
  |    (16) max(3.33, min(sqrt(1 - sin(2 * x) + cos(pi / y) / 3), 1.11)) 
  |    (17) if((y + (x * 2.2)) <= (x + y + 1.1), x - y, x*y) + 2 * pi / x 
  |  
  |  
  | The  third  and  final  phase  (c),  is  used  to  determine   average 
  | compilation rates  (compiles per  second) for  expressions of  varying 
  | complexity. Each expression is compiled 100K times and the average for 
  | each expression is output. 
  |  
  |  
  | (2) User Specified Expressions 
  | In this mode two parameters are passed to the utility via the  command 
  | line: 
  |  
  |    (a) A name of a text file containing one expression per line 
  |    (b) An integer representing the number of evaluations per expression 
  |  
  |  
  | An  example execution  of the  benchmark utility  in this  mode is  as 
  | follows: 
  |  
  |    ./exprtk_benchmark my_expressions.txt 1000000 
  |  
  |  
  | The  above  invocation  will  load  the  expressions  from  the   file 
  | 'my_expressions.txt' and will then proceed to evaluate each expression 
  | one million  times, varying  the above  mentioned variables  (x, y,  z 
  | etc.) between each evaluation, and at the end of each expression round 
  | a print out of  running times, result of a single evaluation and total 
  | sum of results is provided as demonstrated below: 
  |  
  |    Expression 1 of 7 4.770 ns 47700 ns  ( 9370368.0) '((((x+y)+z)))' 
  |    Expression 2 of 7 4.750 ns 47500 ns  ( 1123455.9) '((((x+y)-z)))' 
  |    Expression 3 of 7 4.766 ns 47659 ns  (21635410.7) '((((x+y)*z)))' 
  |    Expression 4 of 7 5.662 ns 56619 ns  ( 1272454.9) '((((x+y)/z)))' 
  |    Expression 5 of 7 4.950 ns 49500 ns  ( 4123455.9) '((((x-y)+z)))' 
  |    Expression 6 of 7 7.581 ns 75810 ns  (-4123455.9) '((((x-y)-z)))' 
  |    Expression 7 of 7 4.801 ns 48010 ns  (       0.0) '((((x-y)*z)))' 
  |  
  |  
  | The benchmark utility can be very useful when investigating evaluation 
  | efficiency  issues with  ExprTk or  simply during  the prototyping  of 
  | expressions. As an example, lets take the following expression: 
  |  
  |    1 / sqrt(2x) * e^(3y) 
  |  
  |  
  | Lets say we would like to determine which sub-part  of the  expression 
  | takes the  most time  to evaluate  and perhaps  attempt to  rework the 
  | expression based on the results. In order to do this we will create  a 
  | text file  called 'test.txt'  and then  proceed to  make some educated 
  | guesses  about  how  to  break   the  expression  up  into  its   more 
  | 'interesting' sub-parts which we will  then add as one expression  per 
  | line to the file. An example breakdown may be as follows: 
  |  
  |    1 / sqrt(2x) * e^(3y) 
  |    1 / sqrt(2x) 
  |    e^(3y) 
  |  
  |  
  | The  benchmark with  the given  file, where  each expression  will be 
  | evaluated 100K times can be executed as follows: 
  |  
  |    ./exprtk_benchmark test.txt 100000 
  |    Expr 1 of 3 90.340 ns 9034000 ns (296417859.3) '1/sqrt(2x)*e^(3y)' 
  |    Expr 2 of 3 11.100 ns 1109999 ns (    44267.3) '1/sqrt(2x)' 
  |    Expr 3 of 3 77.830 ns 7783000 ns (615985286.6) 'e^(3y)' 
  |    [*] Number Of Evals:         300000 
  |    [*] Total Time:              0.018sec 
  |    [*] Total Single Eval Time:  0.000ms 
  |  
  |  
  | From the results above we conclude that the third expression  (e^(3y)) 
  | consumes the largest amount of time. The variable 'e', as used in both 
  | the  benchmark  and  in the  expression,  is  an approximation  of the 
  | transcendental mathematical constant e (2.71828182845904...) hence the 
  | sub-expression  should perhaps be  modified  to use the generally more 
  | efficient built-in 'exp' function. 
  |  
  |    ./exprtk_benchmark test.txt 1000000 
  |    Expr 1 of 5 86.563 ns 8656300ns (296417859.6) '1/sqrt(2x)*e^(3y)' 
  |    Expr 2 of 5 40.506 ns 4050600ns (296417859.6) '1/sqrt(2x)*exp(3y)' 
  |    Expr 3 of 5 14.248 ns 1424799ns (    44267.2) '1/sqrt(2x)' 
  |    Expr 4 of 5 88.840 ns 8884000ns (615985286.9) 'e^(3y)' 
  |    Expr 5 of 5 29.267 ns 2926699ns (615985286.9) 'exp(3y)' 
  |    [*] Number Of Evals:        5000000 
  |    [*] Total Time:             0.260sec 
  |    [*] Total Single Eval Time: 0.000ms 
  |  
  |  
  | The above output demonstrates the  results from  making the previously 
  | mentioned modification to the expression. As can be seen the new  form 
  | of the expression using the 'exp' function reduces the evaluation time 
  | by over 50%, in other words increases the evaluation rate by two fold. 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 26 - EXPRTK NOTES] 
  | The following is a list of facts and suggestions one may want to take 
  | into account when using ExprTk: 
  |  
  |  (00) Precision  and performance  of expression  evaluations are  the 
  |       dominant principles of the ExprTk library. 
  |  
  |  (01) ExprTk  uses a  rudimentary imperative  programming model  with 
  |       syntax based  on languages  such as  Pascal and  C. Furthermore 
  |       ExprTk  is  an LL(2)  type  grammar and  is  processed using  a 
  |       recursive descent parsing algorithm. 
  |  
  |  (02) Supported types  are float,  double, long  double and MPFR/GMP. 
  |       Generally any user defined numerical type that supports all the 
  |       basic floating  point arithmetic  operations: -,+,*,/,^,%  etc; 
  |       unary and binary operations: sin,cos,min,max,equal etc and  any 
  |       other ExprTk dependent operations can be used to specialise the 
  |       various components: expression, parser and symbol_table. 
  |  
  |  (03) Standard arithmetic operator precedence is applied (BEDMAS). In 
  |       general C, Pascal or Rust equivalent unary, binary, logical and 
  |       equality/inequality operator precedence rules apply. 
  |       eg: a == b and c > d + 1  --->  (a == b) and (c > (d + 1)) 
  |           x - y <= z / 2        --->  (x - y) <= (z / 2) 
  |           a - b / c * d^2^3     --->  a - ((b / c) * d^(2^3)) 
  |  
  |  (04) Results of expressions that are deemed as being 'valid' are  to 
  |       exist within the set of Real numbers. All other results will be 
  |       of  the  value:  Not-A-Number  (NaN).  However  this  may   not 
  |       necessarily be a requirement for user defined numerical  types, 
  |       eg: complex number type. 
  |  
  |  (05) Supported   user  defined   types   are   numeric  and   string 
  |       variables, numeric vectors and functions. 
  |  
  |  (06) All  reserved words,  keywords, variable,  vector, string   and 
  |       function names are case-insensitive. 
  |  
  |  (07) Variable, vector, string variable and function names must begin 
  |       with  a letter  (A-Z or  a-z), then  can  be  comprised of  any 
  |       combination of letters, digits, underscores and non-consecutive 
  |       dots,  ending  in  either  a  letter  (A-Z  or  a-z),  digit or 
  |       underscore.  (eg:  x,   y2,  var1,   power_func99,  person.age, 
  |       item.size.0). The associated regex pattern is: 
  |       [a-zA-Z][a-zA-Z0-9_]*(\.[a-zA-Z0-9_]+)* 
  |  
  |  (08) Expression lengths and sub-expression lists are limited only by 
  |       storage capacity. 
  |  
  |  (09) The  life-time of  objects registered  with or  created from  a 
  |       specific symbol-table must  span at least  the lifetime of  the 
  |       symbol  table  instance  and  all  compiled  expressions  which 
  |       utilise objects, such as variables, strings, vectors,  function 
  |       compositor  functions  and  functions  of  that   symbol-table, 
  |       otherwise the result will be undefined behaviour. 
  |  
  |  (10) Equal and not_equal  are  normalised-epsilon equality routines, 
  |       which use epsilons of 0.0000000001 and 0.000001 for double  and 
  |       float types respectively. 
  |  
  |  (11) All trigonometric functions  assume radian input  unless stated 
  |       otherwise. 
  |  
  |  (12) Expressions may contain  white-space characters such  as space, 
  |       tabs, new-lines, control-feed et al. 
  |       ('\n', '\r', '\t', '\b', '\v', '\f') 
  |  
  |  (13) Strings may be comprised of any  combination of letters, digits 
  |       special characters including (~!@#$%^&*()[]|=+ ,./?<>;:"`~_) or 
  |       hexadecimal escaped sequences (eg: \0x30) and must be enclosed 
  |       with single-quotes. 
  |       eg: 'Frankly my dear, \0x49 do n0t give a damn!' 
  |  
  |  (14) User defined  normal functions  can have  up to  20 parameters, 
  |       where as  user defined  generic-functions and  vararg-functions 
  |       can have an unlimited number of parameters. 
  |  
  |  (15) The inbuilt polynomial functions can be at most of degree 12. 
  |  
  |  (16) Where appropriate constant folding optimisations may be applied. 
  |       (eg: The expression '2 + (3 - (x / y))' becomes '5 - (x / y)') 
  |  
  |  (17) If the strength  reduction compilation option has  been enabled, 
  |       then where applicable  strength reduction  optimisations  may be 
  |       applied. 
  |  
  |  (18) String  processing capabilities  are available  by default.  To 
  |       turn them  off, the  following needs  to be  defined at compile 
  |       time: exprtk_disable_string_capabilities 
  |  
  |  (19) Composited functions can call themselves or any other functions 
  |       that have been defined prior to their own definition. 
  |  
  |  (20) Recursive calls made from within composited functions will have 
  |       a stack size bound by the stack of the executing architecture. 
  |  
  |  (21) User  defined functions  by default  are assumed  to have  side 
  |       effects. As such an "all constant parameter" invocation of such 
  |       functions wont result in constant folding. If the function  has 
  |       no side-effects then that  can be noted during the  constructor 
  |       of  the  ifunction  allowing it  to  be  constant folded  where 
  |       appropriate. 
  |  
  |  (22) The entity relationship between symbol_table and an  expression 
  |       is many-to-many. However the intended 'typical' use-case  where 
  |       possible, is to have a single symbol table manage the  variable 
  |       and function requirements of multiple expressions. 
  |  
  |  (23) The common use-case  for an expression  is to have  it compiled 
  |       only  ONCE  and  then subsequently  have it  evaluated multiple 
  |       times. An extremely  inefficient and suboptimal  approach would 
  |       be to recompile an expression  from its string form every  time 
  |       it requires evaluating. 
  |  
  |  (24) It is  strongly recommended  that  the  return value  of method 
  |       invocations from  the parser  and symbol_table  types be  taken 
  |       into account. Specifically the  'compile' method of the  parser 
  |       and the 'add_xxx'  set of methods  of the symbol_table  as they 
  |       denote either the success or failure state of the invoked call. 
  |       Continued processing from  a failed  state without having first 
  |       rectified the underlying issue  will in turn result  in further 
  |       failures and undefined behaviours. 
  |  
  |  (25) The following are examples of compliant floating point value 
  |       representations: 
  |  
  |       (01) 12345         (06) -123.456 
  |       (02) +123.456e+12  (07) 123.456E-12 
  |       (03) +012.045e+07  (08) .1234 
  |       (04) 1234.         (09) -56789. 
  |       (05) 123.456f      (10) -321.654E+3L 
  |  
  |  (26) Expressions may contain any of the following comment styles: 
  |  
  |       (1) // .... \n 
  |       (2) #  .... \n 
  |       (3) /* .... */ 
  |  
  |  (27) The  'null'  value  type  is  a  special  non-zero  type   that 
  |       incorporates specific semantics when undergoing operations with 
  |       the standard numeric type. The following is a list of type  and 
  |       boolean results associated with the use of 'null': 
  |  
  |       (1) null  +,-,*,/,%  x    --> x 
  |       (2) x     +,-,*,/,%  null --> x 
  |       (3) null  +,-,*,/,%  null --> null 
  |       (4) null     ==      null --> true 
  |       (5) null     ==      x    --> true 
  |       (6) x        ==      null --> true 
  |       (7) x        !=      null --> false 
  |       (8) null     !=      null --> false 
  |       (9) null     !=      x    --> false 
  |  
  |  (28) The following is a list  of reserved words and symbols  used by 
  |       ExprTk. Attempting to  add a variable  or custom function  to a 
  |       symbol table using any of  the reserved words will result  in a 
  |       failure. 
  |  
  |       abs, acos,  acosh, and, asin,  asinh,  assert, atan,  atan2, 
  |       atanh, avg, break, case,  ceil, clamp, continue, cosh,  cos, 
  |       cot,  csc, default,  deg2grad, deg2rad,  else, equal,  erfc, 
  |       erf, exp, expm1, false,  floor, for, frac, grad2deg,  hypot, 
  |       iclamp, if, ilike, in, inrange, in, like, log, log10, log1p, 
  |       log2, logn, mand, max, min, mod, mor, mul, nand, ncdf,  nor, 
  |       not, not_equal, not, null, or, pow, rad2deg, repeat, return, 
  |       root, roundn, round,  sec, sgn, shl,  shr, sinc, sinh,  sin, 
  |       sqrt, sum, swap, switch, tanh, tan, true, trunc, until, var, 
  |       while, xnor, xor 
  |  
  |  (29) Every valid ExprTk statement is a "value returning" expression. 
  |       Unlike some languages that limit the types of expressions  that 
  |       can be  performed in  certain situations,  in ExprTk  any valid 
  |       expression can be used in any "value consuming" context. eg: 
  |  
  |       var y := 3; 
  |       for (var x := switch 
  |                     { 
  |                       case 1  :  7; 
  |                       case 2  : -1 + ~{var x{};}; 
  |                       default :  y > 2 ? 3 : 4; 
  |                     }; 
  |            x != while (y > 0) { y -= 1; }; 
  |            x -= { 
  |                   if (min(x,y) < 2 * max(x,y)) 
  |                     x + 2; 
  |                   else 
  |                     x + y - 3; 
  |                 } 
  |           ) 
  |       { 
  |         (x + y) / (x - y); 
  |       }; 
  |  
  |  (30) It is recommended when prototyping expressions that the  ExprTk 
  |       REPL be utilised, as it supports all the features available  in 
  |       the library,  including complete  error analysis,  benchmarking 
  |       and   dependency   dumps    etc   which   allows    for   rapid 
  |       coding/prototyping  and  debug  cycles  without  the  hassle of 
  |       having to  recompile test  programs with  expressions that have 
  |       been hard-coded. It is also a  good source of truth for how the 
  |       library's various features can be applied. 
  |  
  |  (31) For performance considerations,  one should assume  the actions 
  |       of expression, symbol  table and parser  instance instantiation 
  |       and destruction, and the expression compilation process  itself 
  |       to be of high latency. Hence none of them should be part of any 
  |       performance  critical  code  paths, and  should  instead  occur 
  |       entirely either before or after such code paths. 
  |  
  |  (32) Deep  copying  an  expression  instance  for  the  purposes  of 
  |       persisting to disk or otherwise transmitting elsewhere with the 
  |       intent to 'resurrect' the  expression instance later on  is not 
  |       possible due  to the  reasons described  in the  final note  of 
  |       Section 10. The recommendation is to instead simply persist the 
  |       string form  of the  expression and  compile the  expression at 
  |       run-time on the target. 
  |  
  |  (33) The  correctness  and  robustness  of  the  ExprTk  library  is 
  |       maintained by having  a comprehensive suite  of unit tests  and 
  |       functional tests all of  which are run using  sanitizers (ASAN, 
  |       UBSAN, LSAN, MSAN, TSAN). Additionally, continuous fuzz-testing 
  |       provided by Google  OSS Fuzz, and static analysis via  Synopsis 
  |       Coverity. 
  |  
  |  (34) The library  name ExprTk  is pronounced  "Ex-Pee-Ar-Tee-Kay" or 
  |       simply "Mathematical Expression Toolkit" 
  |  
  |  
  |  (35) For general support, inquires or bug/issue reporting: 
  |       https://www.partow.net/programming/exprtk/index.html#support 
  |  
  |  (36) Before jumping in and using ExprTk, do take the time to  peruse 
  |       the documentation and all of the examples, both in the main and 
  |       the extras  distributions. Having  an informed  general view of 
  |       what can and  can't be done,  and how something  should be done 
  |       with ExprTk, will  likely result in  a far more  productive and 
  |       enjoyable programming experience. 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 27 - SIMPLE EXPRTK EXAMPLE] 
  | The following is a  simple yet complete example  demonstrating typical 
  | usage of the ExprTk Library.  The example instantiates a symbol  table 
  | object, adding to it  three variables named x,  y and z, and  a custom 
  | user defined function, that accepts only two parameters, named myfunc. 
  | The  example then  proceeds to  instantiate an  expression object  and 
  | register to it the symbol table instance. 
  |  
  | A parser is  then instantiated, and  the string representation  of the 
  | expression  and  the  expression object  are  passed  to the  parser's 
  | compile  method   for  compilation.   If  an   error  occurred  during 
  | compilation, the compile method will return false, leading to a series 
  | of  error diagnostics  being printed  to stdout.  Otherwise the  newly 
  | compiled expression is evaluated  by invoking  the expression object's 
  | value method, and subsequently printing the result  of the computation 
  | to stdout. 
  |  
  |  
  | --- snip --- 
  | #include  
  | #include  
  |  
  | #include "exprtk.hpp" 
  |  
  | template  
  | struct myfunc final : public exprtk::ifunction<T> 
  | { 
  |    myfunc() : exprtk::ifunction<T>(2) {} 
  |  
  |    T operator()(const T& v1, const T& v2) override 
  |    { 
  |       return T(1) + (v1 * v2) / T(3); 
  |    } 
  | }; 
  |  
  | int main() 
  | { 
  |    typedef exprtk::symbol_table<double> symbol_table_t; 
  |    typedef exprtk::expression<double>   expression_t; 
  |    typedef exprtk::parser<double>       parser_t; 
  |    typedef exprtk::parser_error::type   error_t; 
  |  
  |    const std::string expression_string = 
  |       "z := 2 myfunc([4 + sin(x / pi)^3],y ^ 2)" 
  |  
  |    double x = 1.1; 
  |    double y = 2.2; 
  |    double z = 3.3; 
  |  
  |    myfunc<double> mf; 
  |  
  |    symbol_table_t symbol_table; 
  |    symbol_table.add_constants(); 
  |    symbol_table.add_variable("x",x); 
  |    symbol_table.add_variable("y",y); 
  |    symbol_table.add_variable("z",z); 
  |    symbol_table.add_function("myfunc",mf); 
  |  
  |    expression_t expression; 
  |    expression.register_symbol_table(symbol_table); 
  |  
  |    parser_t parser; 
  |  
  |    if (!parser.compile(expression_string,expression)) 
  |    { 
  |       // A compilation error has occurred. Attempt to 
  |       // print all errors to stdout. 
  |  
  |       printf("Error: %s\tExpression: %s\n", 
  |              parser.error().c_str(), 
  |              expression_string.c_str()); 
  |  
  |       for (std::size_t i = 0; i < parser.error_count(); ++i) 
  |       { 
  |          // Include the specific nature of each error 
  |          // and its position in the expression string. 
  |  
  |          error_t error = parser.get_error(i); 
  |  
  |          printf("Error: %02d Position: %02d " 
  |                 "Type: [%s] " 
  |                 "Message: %s " 
  |                 "Expression: %s\n", 
  |                 static_cast<int>(i), 
  |                 static_cast<int>(error.token.position), 
  |                 exprtk::parser_error::to_str(error.mode).c_str(), 
  |                 error.diagnostic.c_str(), 
  |                 expression_string.c_str()); 
  |       } 
  |  
  |       return 1; 
  |    } 
  |  
  |    // Evaluate the expression and obtain its result. 
  |  
  |    double result = expression.value(); 
  |  
  |    printf("Result: %10.5f\n",result); 
  |  
  |    return 0; 
  | } 
  | --- snip --- 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 28 - BUILD OPTIONS] 
  | When building ExprTk there are a number of defines that will enable or 
  | disable certain features and  capabilities. The defines can  either be 
  | part of a compiler command line switch or scoped around the include to 
  | the ExprTk header. The defines are as follows: 
  |  
  |    (01) exprtk_enable_debugging 
  |    (02) exprtk_disable_cardinal_pow_optimisation 
  |    (03) exprtk_disable_comments 
  |    (04) exprtk_disable_break_continue 
  |    (05) exprtk_disable_sc_andor 
  |    (06) exprtk_disable_return_statement 
  |    (07) exprtk_disable_enhanced_features 
  |    (08) exprtk_disable_string_capabilities 
  |    (09) exprtk_disable_superscalar_unroll 
  |    (10) exprtk_disable_rtl_io 
  |    (11) exprtk_disable_rtl_io_file 
  |    (12) exprtk_disable_rtl_vecops 
  |    (13) exprtk_disable_caseinsensitivity 
  |    (14) exprtk_enable_range_runtime_checks 
  |  
  | (01) exprtk_enable_debugging 
  | This define will enable printing of debug information to stdout during 
  | the compilation process. 
  |  
  | (02) exprtk_disable_cardinal_pow_optimisation 
  | This  define   will  disable  the optimisation  invoked when  constant 
  | integers are used as powers in exponentiation expressions (eg: x^7). 
  |  
  | (03) exprtk_disable_comments 
  | This define will disable the ability for expressions to have comments. 
  | Expressions that have comments when parsed with a build that has  this 
  | option, will result in a compilation failure. 
  |  
  | (04) exprtk_disable_break_continue 
  | This  define  will  disable  the  loop-wise  'break'  and   'continue' 
  | capabilities. Any expression that contains those keywords will  result 
  | in a compilation failure. 
  |  
  | (05) exprtk_disable_sc_andor 
  | This define  will disable  the short-circuit  '&' (and)  and '|'  (or) 
  | operators 
  |  
  | (06) exprtk_disable_return_statement 
  | This define will disable use of return statements within expressions. 
  |  
  | (07) exprtk_disable_enhanced_features 
  | This  define  will  disable all  enhanced  features  such as  strength 
  | reduction and special  function optimisations and  expression specific 
  | type instantiations.  This feature  will reduce  compilation times and 
  | binary sizes but will  also result in massive  performance degradation 
  | of expression evaluations. 
  |  
  | (08) exprtk_disable_string_capabilities 
  | This  define  will  disable all  string  processing  capabilities. Any 
  | expression that contains a string or string related syntax will result 
  | in a compilation failure. 
  |  
  | (09) exprtk_disable_superscalar_unroll 
  | This define will set  the loop unroll batch  size to 4 operations  per 
  | loop  instead of  the default  8 operations.  This define  is used  in 
  | operations that  involve vectors  and aggregations  over vectors. When 
  | targeting  non-superscalar  architectures, it  may  be recommended  to 
  | build using this particular option if efficiency of evaluations is  of 
  | concern. 
  |  
  | (10) exprtk_disable_rtl_io 
  | This define will  disable all of  basic IO RTL  package features. When 
  | present, any attempt to register the basic IO RTL package with a given 
  | symbol table will fail causing a compilation error. 
  |  
  | (11) exprtk_disable_rtl_io_file 
  | This  define will  disable  the  file I/O  RTL package  features. When 
  | present, any  attempts to register  the file I/O package with  a given 
  | symbol table will fail causing a compilation error. 
  |  
  | (12) exprtk_disable_rtl_vecops 
  | This define will  disable the extended  vector operations RTL  package 
  | features. When present, any attempts to register the vector operations 
  | package with  a given  symbol table  will fail  causing a  compilation 
  | error. 
  |  
  | (13) exprtk_disable_caseinsensitivity 
  | This define  will disable  case-insensitivity when  matching variables 
  | and  functions. Furthermore  all reserved  and keywords  will only  be 
  | acknowledged when in all lower-case. 
  |  
  | (14) exprtk_enable_range_runtime_checks 
  | This define will enable run-time checks pertaining to vector  indexing 
  | operations used  in any  of the  vector-to-vector and vector-to-scalar 
  | operations. 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 29 - FILES] 
  | The source distribution of ExprTk is comprised of the following set of 
  | files: 
  |  
  |    (00) Makefile 
  |    (01) readme.txt 
  |    (02) exprtk.hpp 
  |    (03) exprtk_test.cpp 
  |    (04) exprtk_benchmark.cpp 
  |    (05) exprtk_simple_example_01.cpp 
  |    (06) exprtk_simple_example_02.cpp 
  |    (07) exprtk_simple_example_03.cpp 
  |    (08) exprtk_simple_example_04.cpp 
  |    (09) exprtk_simple_example_05.cpp 
  |    (10) exprtk_simple_example_06.cpp 
  |    (11) exprtk_simple_example_07.cpp 
  |    (12) exprtk_simple_example_08.cpp 
  |    (13) exprtk_simple_example_09.cpp 
  |    (14) exprtk_simple_example_10.cpp 
  |    (15) exprtk_simple_example_11.cpp 
  |    (16) exprtk_simple_example_12.cpp 
  |    (17) exprtk_simple_example_13.cpp 
  |    (18) exprtk_simple_example_14.cpp 
  |    (19) exprtk_simple_example_15.cpp 
  |    (20) exprtk_simple_example_16.cpp 
  |    (21) exprtk_simple_example_17.cpp 
  |    (22) exprtk_simple_example_18.cpp 
  |    (23) exprtk_simple_example_19.cpp 
  |    (24) exprtk_simple_example_20.cpp 
  |    (25) exprtk_simple_example_21.cpp 
  |    (26) exprtk_simple_example_22.cpp 
  |    (27) exprtk_simple_example_23.cpp 
  |    (28) exprtk_simple_example_24.cpp 
  |  
  |  
  | Details for each of the above examples can be found here: 
  |  
  |   https://www.partow.net/programming/exprtk/index.html#examples 
  |  
  |  
  | Various extended and advanced examples using ExprTk are available 
  | via the following: 
  |  
  |    (00) exprtk_american_option_binomial_model.cpp 
  |    (01) exprtk_archimedes_pi.cpp 
  |    (02) exprtk_arithmetic_evaluator.cpp 
  |    (03) exprtk_binomial_coefficient.cpp 
  |    (04) exprtk_bsm_benchmark.cpp 
  |    (05) exprtk_calc.cpp 
  |    (06) exprtk_collatz.cpp 
  |    (07) exprtk_compilation_timeout.cpp 
  |    (08) exprtk_degree_trigonometry_example.cpp 
  |    (09) exprtk_exprgen.cpp 
  |    (00) exprtk_extract_dependents.cpp 
  |    (11) exprtk_e_10kdigits.cpp 
  |    (12) exprtk_factorize_fermat.cpp 
  |    (13) exprtk_factorize_pollard.cpp 
  |    (14) exprtk_fizzbuzz.cpp 
  |    (15) exprtk_funcall_benchmark.cpp 
  |    (16) exprtk_game_of_life.cpp 
  |    (17) exprtk_gcd.cpp 
  |    (18) exprtk_gnuplot.cpp 
  |    (19) exprtk_gnuplot_multi.cpp 
  |    (10) exprtk_groups_examples.cpp 
  |    (21) exprtk_immutable_symbol_table_example.cpp 
  |    (22) exprtk_import_packages.cpp 
  |    (23) exprtk_instruction_primer.cpp 
  |    (24) exprtk_jump_diffusion_process.cpp 
  |    (25) exprtk_loop_timeout_rtc.cpp 
  |    (26) exprtk_magic_square.cpp 
  |    (27) exprtk_mandelbrot.cpp 
  |    (28) exprtk_max_subarray_sum.cpp 
  |    (29) exprtk_maze_generator.cpp 
  |    (20) exprtk_miller_rabin_primality_test.cpp 
  |    (31) exprtk_montecarlo_e.cpp 
  |    (32) exprtk_montecarlo_option_pricing_model.cpp 
  |    (33) exprtk_montecarlo_pi.cpp 
  |    (34) exprtk_naive_primes.cpp 
  |    (35) exprtk_normal_random_marsaglia_method.cpp 
  |    (36) exprtk_nqueens_problem.cpp 
  |    (37) exprtk_nthroot_bisection.cpp 
  |    (38) exprtk_ornstein_uhlenbeck_process.cpp 
  |    (39) exprtk_pascals_triangle.cpp 
  |    (30) exprtk_pi_10kdigits.cpp 
  |    (41) exprtk_prime_sieve.cpp 
  |    (42) exprtk_prime_sieve_vectorized.cpp 
  |    (43) exprtk_pyramid.cpp 
  |    (44) exprtk_pythagorean_triples.cpp 
  |    (45) exprtk_recursive_fibonacci.cpp 
  |    (46) exprtk_repl.cpp 
  |    (47) exprtk_riddle.cpp 
  |    (48) exprtk_rtc_overhead.cpp 
  |    (49) exprtk_sudoku_solver.cpp 
  |    (50) exprtk_sumofprimes.cpp 
  |    (51) exprtk_symtab_functions.cpp 
  |    (52) exprtk_testgen.cpp 
  |    (53) exprtk_tower_of_hanoi.cpp 
  |    (54) exprtk_truthtable_gen.cpp 
  |    (55) exprtk_vectorized_binomial_model.cpp 
  |    (56) exprtk_vectornorm.cpp 
  |    (57) exprtk_vector_benchmark.cpp 
  |    (58) exprtk_vector_benchmark_multithreaded.cpp 
  |    (59) exprtk_vector_resize_example.cpp 
  |    (60) exprtk_vector_resize_inline_example.cpp 
  |    (61) exprtk_wiener_process_pi.cpp 
  |  
  |  
  | Details for each of the above examples can be found here: 
  |  
  |    https://partow.net/programming/exprtk/index.html#variousexamples 
  |  
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
  |  
  | [SECTION 30 - LANGUAGE STRUCTURE] 
  | The following  are the  various language  structures available  within 
  | ExprTk and their structural representations. 
  |  
  |    (00) If Statement 
  |    (01) Else Statement 
  |    (02) Ternary Statement 
  |    (03) While Loop 
  |    (04) Repeat Until Loop 
  |    (05) For Loop 
  |    (06) Switch Statement 
  |    (07) Multi Subexpression Statement 
  |    (08) Multi Case-Consequent Statement 
  |    (09) Variable Definition Statement 
  |    (10) Vector Definition Statement 
  |    (11) String Definition Statement 
  |    (12) Range Statement 
  |    (13) Return Statement 
  |  
  |  
  | (00) - If Statement 
  | +-------------------------------------------------------------+ 
  | |                                                             | 
  | |   [if] ---> [(] ---> [condition] -+-> [,] -+                | 
  | |                                   |        |                | 
  | |   +---------------<---------------+        |                | 
  | |   |                                        |                | 
  | |   |  +------------------<------------------+                | 
  | |   |  |                                                      | 
  | |   |  +--> [consequent] ---> [,] ---> [alternative] ---> [)] | 
  | |   |                                                         | 
  | |   +--> [)] --+-> [{] ---> [expression*] ---> [}] --+        | 
  | |              |                                     |        | 
  | |              |                +---------<----------+        | 
  | |   +----<-----+                |                             | 
  | |   |                           v                             | 
  | |   +--> [consequent] --> [;] -{*}-> [else-statement]         | 
  | |                                                             | 
  | +-------------------------------------------------------------+ 
  |  
  |  
  | (01) - Else Statement 
  | +-------------------------------------------------------------+ 
  | |                                                             | 
  | |   [else] -+-> [alternative] ---> [;]                        | 
  | |           |                                                 | 
  | |           +--> [{] ---> [expression*] ---> [}]              | 
  | |           |                                                 | 
  | |           +--> [if-statement]                               | 
  | |                                                             | 
  | +-------------------------------------------------------------+ 
  |  
  |  
  | (02) - Ternary Statement 
  | +-------------------------------------------------------------+ 
  | |                                                             | 
  | |   [condition] ---> [?] ---> [consequent] ---> [:] --+       | 
  | |                                                     |       | 
  | |   +------------------------<------------------------+       | 
  | |   |                                                         | 
  | |   +--> [alternative] --> [;]                                | 
  | |                                                             | 
  | +-------------------------------------------------------------+ 
  |  
  |  
  | (03) - While Loop 
  | +-------------------------------------------------------------+ 
  | |                                                             | 
  | |   [while] ---> [(] ---> [condition] ---> [)] ---+           | 
  | |                                                 |           | 
  | |   +----------------------<----------------------+           | 
  | |   |                                                         | 
  | |   +--> [{] ---> [expression*] ---> [}]                      | 
  | |                                                             | 
  | +-------------------------------------------------------------+ 
  |  
  |  
  | (04) - Repeat Until Loop 
  | +-------------------------------------------------------------+ 
  | |                                                             | 
  | |   [repeat] ---> [expression*] ---+                          | 
  | |                                  |                          | 
  | |   +--------------<---------------+                          | 
  | |   |                                                         | 
  | |   +--> [until] ---> [(] ---> [condition] --->[)]            | 
  | |                                                             | 
  | +-------------------------------------------------------------+ 
  |  
  |  
  | (05) - For Loop 
  | +-------------------------------------------------------------+ 
  | |                                                             | 
  | |   [for] ---> [(] -+-> [initialise expression] --+--+        | 
  | |                   |                             |  |        | 
  | |                   +------------->---------------+  v        | 
  | |                                                    |        | 
  | |   +-----------------------<------------------------+        | 
  | |   |                                                         | 
  | |   +--> [;] -+-> [condition] -+-> [;] ---+                   | 
  | |             |                |          |                   | 
  | |             +------->--------+          v                   | 
  | |                                         |                   | 
  | |   +------------------<---------+--------+                   | 
  | |   |                            |                            | 
  | |   +--> [increment expression] -+-> [)] --+                  | 
  | |                                          |                  | 
  | |   +------------------<-------------------+                  | 
  | |   |                                                         | 
  | |   +--> [{] ---> [expression*] ---> [}]                      | 
  | |                                                             | 
  | +-------------------------------------------------------------+ 
  |  
  |  
  | (06) - Switch Statement 
  | +-------------------------------------------------------------+ 
  | |                                                             | 
  | |   [switch] ---> [{] ---+                                    | 
  | |                        |                                    | 
  | |   +---------<----------+-----------<-----------+            | 
  | |   |                                            |            | 
  | |   +--> [case] ---> [condition] ---> [:] ---+   |            | 
  | |                                            |   |            | 
  | |   +-------------------<--------------------+   |            | 
  | |   |                                            |            | 
  | |   +--> [consequent] ---> [;] --------->--------+            | 
  | |   |                                            |            | 
  | |   |                                            |            | 
  | |   +--> [default] ---> [consequent] ---> [;] ---+            | 
  | |   |                                            |            | 
  | |   +---------------------<----------------------+            | 
  | |   |                                                         | 
  | |   +--> [}]                                                  | 
  | |                                                             | 
  | +-------------------------------------------------------------+ 
  |  
  |  
  | (07) - Multi Subexpression Statement 
  | +-------------------------------------------------------------+ 
  | |                                                             | 
  | |                   +--------------<---------------+          | 
  | |                   |                              |          | 
  | |   [~] ---> [{\(] -+-> [expression] -+-> [;\,] ---+          | 
  | |                                     |                       | 
  | |   +----------------<----------------+                       | 
  | |   |                                                         | 
  | |   +--> [}\)]                                                | 
  | |                                                             | 
  | +-------------------------------------------------------------+ 
  |  
  |  
  | (08) - Multi Case-Consequent Statement 
  | +-------------------------------------------------------------+ 
  | |                                                             | 
  | |   [[*]] ---> [{] ---+                                       | 
  | |                     |                                       | 
  | |   +--------<--------+--------------<----------+             | 
  | |   |                                           |             | 
  | |   +--> [case] ---> [condition] ---> [:] ---+  |             | 
  | |                                            |  |             | 
  | |   +-------------------<--------------------+  |             | 
  | |   |                                           |             | 
  | |   +--> [consequent] ---> [;] ---+------>------+             | 
  | |                                 |                           | 
  | |                                 +--> [}]                    | 
  | |                                                             | 
  | +-------------------------------------------------------------+ 
  |  
  |  
  | (09) - Variable Definition Statement 
  | +-------------------------------------------------------------+ 
  | |                                                             | 
  | |   [var] ---> [symbol] -+-> [:=] -+-> [expression] -+-> [;]  | 
  | |                        |         |                 |        | 
  | |                        |         +-----> [{}] -->--+        | 
  | |                        |                           |        | 
  | |                        +------------->-------------+        | 
  | |                                                             | 
  | +-------------------------------------------------------------+ 
  |  
  |  
  | (10) - Vector Definition Statement 
  | +-------------------------------------------------------------+ 
  | |                                                             | 
  | |   [var] ---> [symbol] ---> [[] ---> [constant] ---> []] --+ | 
  | |                                                           | | 
  | |   +---------------------------<---------------------------+ | 
  | |   |                                                         | 
  | |   |                   +--------->---------+                 | 
  | |   |                   |                   |                 | 
  | |   +--> [:=] ---> [{] -+-+-> [expression] -+-> [}] ---> [;]  | 
  | |                         |                 |                 | 
  | |                         +--<--- [,] <-----+                 | 
  | |                                                             | 
  | +-------------------------------------------------------------+ 
  |  
  |  
  | (11) - String Definition Statement 
  | +-------------------------------------------------------------+ 
  | |                                                             | 
  | |   [var] --> [symbol] --> [:=] --> [str-expression] ---> [;] | 
  | |                                                             | 
  | +-------------------------------------------------------------+ 
  |  
  |  
  | (12) - Range Statement 
  | +-------------------------------------------------------------+ 
  | |                                                             | 
  | |      +-------->--------+                                    | 
  | |      |                 |                                    | 
  | | [[] -+-> [expression] -+-> [:] -+-> [expression] -+--> []]  | 
  | |                                 |                 |         | 
  | |                                 +-------->--------+         | 
  | |                                                             | 
  | +-------------------------------------------------------------+ 
  |  
  |  
  | (13) - Return Statement 
  | +-------------------------------------------------------------+ 
  | |                                                             | 
  | |   [return] ---> [[] -+-> [expression] -+-> []] ---> [;]     | 
  | |                      |                 |                    | 
  | |                      +--<--- [,] <-----+                    | 
  | |                                                             | 
  | +-------------------------------------------------------------+