Add NA function

Just an alias for NAN, added for parity with Excel

Author: Blake-Madden

docs/manual/functions.qmd

@@ -29,7 +29,8 @@ The following built-in functions are available:
 | MAX(Number1, Number2, ...) | Returns the largest value from a specified range of values. |
 | MAXINT() | Returns the largest integer that the parser can process. |
 | MOD(Number, Divisor) | Returns the remainder after *Number* is divided by *Divisor*. The result has the same sign as divisor. |
-| NAN | Returns an invalid value (i.e., Not-a-number). |
+| NA  | Returns an invalid value (i.e., Not-a-number). |
+| NAN | Alias for `NA()`. |
 | NCR(Number, NumberChosen) | Alias for `COMBIN()`. |
 | NPR(Number, NumberChosen) | Alias for `PERMUT()`. |
 | ODD(Number) | Returns *Number* rounded up to the nearest odd integer.<br>\linebreak Values are always rounded away from zero (e.g., `ODD(-4)` = -5). |

tinyexpr.cpp

@@ -1466,6 +1466,7 @@ const std::set<te_variable> te_parser::m_functions = { // NOLINT
     { "min", static_cast<te_fun24>(te_builtins::te_min),
       static_cast<te_variable_flags>(TE_PURE | TE_VARIADIC) },
     { "mod", static_cast<te_fun2>(te_builtins::te_modulus), TE_PURE },
+    { "na", static_cast<te_fun0>(te_builtins::te_nan_value), TE_PURE },
     { "nan", static_cast<te_fun0>(te_builtins::te_nan_value), TE_PURE },
     { "ncr", static_cast<te_fun2>(te_builtins::te_ncr), TE_PURE },
     { "nominal", static_cast<te_fun2>(te_builtins::te_nominal), TE_PURE },