(define (apply op stack)
  (cond ((eq? op '+) 
         (cons (+ (car stack)(cadr stack)) (cddr stack)))
        ((eq? op '-)
         (cons (- (car stack)(cadr stack))(cddr stack)))
        ((eq? op '*)
         (cons (* (car stack)(cadr stack))(cddr stack)))
        ((eq? op '/)
         (cons (/ (car stack)(cadr stack))(cddr stack)))))

(define (isOp? sym)
  (or (eq? sym '+)(eq? sym '-)(eq? sym '*)(eq? sym '/)))
                
(define (eval expr)
  (cond ((null? expr) (list))
        ((isOp? (car expr))
             (apply (car expr) (eval (cdr expr))))   ;; reduce stack
        (else (cons (car expr)(eval (cdr expr))))))  ;; push value

(eval (list 0))
(eval (list '+ 4 5))
(eval (list '+ '- 4 3 '* 2 '/ 6 3))