; this uses the CLOS: Common LISP Object System
; First define a new class of objects called STACK
(setq STACK (send Class :new nil))
; send the STACK class instructions on how to do methods/operations
(send STACK :isnew '(data head)) ; constructor describes two attributes
(send STACK :answer :clear nil '((setq data nil))
)
(send STACK :answer :pop nil
    '((setq head (car data)) 
      (setq data (cdr data))
      head
     )
)
(send STACK :answer :push '(new)
                        '((setq data (cons new data)))
)
(send STACK :answer :top nil '((car data)))
(send STACK :answer :empty nil '((null data)))

; some utilities used for unit testing
(define (showdo c) (PROGN () (print C) (eval C)(print '-------)) )
(define (showval c) (PROGN () (prin1 C) (prin1 '=) (print (eval C))
		(print '-------)) )

; tests
(showdo '(send STACK :show))
(setq mystack (send STACK :new))
(showdo '(send mystack :show))
(showdo '(send mystack :clear))
(showval '(send mystack :empty))
(showdo '(send mystack :push 1))
(showval '(send mystack :empty))
(showdo '(send mystack :show))
(showdo '(send mystack :push 2))
(showval '(send mystack :top))
(showval '(send mystack :pop) )
(showval '(send mystack :pop) )
(showdo '(send mystack :show))