StackPop()

         Removes a stack element

    Syntax:

         StackPop( aStack ) -> xOldHead

         aStack : The stack created with StackNew()

    Description:

         Removes a stack element and returns this elements value. The
         stack head comes to be the previously returned.

    Return:

         The older stack head value.

    Example:

         aStack  := StackNew ()
         StackPush ( aStack, "hello world" )
         StackPush ( aStack, "Bye bye" )
         ? StackPop ( aStack )           // "Bye bye"
         ? StackHead( aStack )           // "hello world"