; evaluate post fix notation from page 185 of Cooke
(defun ev(X S); X is the expression and S is the stack
 (cond
  ((null X) S); when expression is empty return the stack 
  ((integerp (car X)) (ev (cdr X) (push(car X) S)) )
  (t  
    (ev (cdr X) 
        (push 
	 (eval (list (car X) (top(pop S)) (top S))) 
         (pop(pop S))
        );/push
     );/ev
  );/t
 );/cond
)
; stack operations
(defun push(X S) (cons X S))
(defun pop(S) (if (null S) NIL (cdr S)))
(defun top(S) (if (null S) NIL (car S)))

; testing
(trace ev)
(print '(ev '(1 2 + 3 *) NIL))
(print (ev '(1 2 + 3 *) NIL))