; defines an object that fits the stack ADT
; usage: (new), (top), (push it), (pop), ...

(let ((stack nil)) ; local variable holds the stack
 (defun new () (setq stack nil)) ; defun functions have global scope
 (defun top () (car stack))
 (defun push (item) (setq stack (cons item stack)))
 (defun pop () (setq stack (cdr stack)))
 (defun empty () (null stack))
)