Stack.prg
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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.

   StackGetTop( <aStack> ) --> value

   Retrieve top stack element without removing.

   StackTop( <aStack> ) --> value

   Retrieve top stack element without removing.