This function lets you modify the output returned by EvalStr or EvaluateStr. The definition may contain one or more
of the following:
Datatype: If you specify a data type, then the modification will affect only the evaluation result of expressions
that evaluate to the specified data type.
InsertAt: By default, if you define multiple outputs, then the one defined most recently is applied first, followed by
the other outputs in reverse order in which they were defined, ending with the output that was defined first. You may
specify a different insertion point than the default with this parameter.
Expression: This represents what the modified output should look like. The expression consists of an identifier
(any temporary variable name) , followed by an equal sign, followed by a string expression representing the value
that will be returned. The variable holds the original value that would have been returned.