StackPush()

         Introduces an element into the stack

    Syntax:

         StackPush( aStack, xValue ) -> Nil

         aStack : The created stack with StackNew()
         xValue : The new value to introduce into the stack

    Description:

         Introduces a new element into the stack and places it as stack
         head.

    Return:

         Nothing.

    Example:

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