Stack.prg
 Functions to implement a stack data structure
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     StackNew( ) --> aStack

     Create a new array.

     StackPush( <aStack>, <exp> ) --> aStack

     Add a new value to a stack array.

     StackPop( <aStack> ) --> value

     Remove the last element of a stack array.

     StackIsEmpty( <aStack> ) --> lEmpty

     Determine if a stack array has no elements.

     StackTop( <aStack> ) --> value

     Retrieve top stack element without removing.