(define (pop stack)
  (let ((var (car stack))
		(ret-stack (cdr stack)))
	(values var ret-stack)))

(define (push var stack)
  (append (list var) stack))

(define (dup stack)
  (let ((head (car stack)))
	(append (list head) stack)))

(define (fact x)
  (define (fact-iter n current)
	(if (= n 1)
	  current
	  (fact-iter (- n 1) (* n current))))
  (fact-iter x 1))

(define (rpn-func func stack)
  (let*-values (((var1 stack) (pop stack))
				((var2 stack) (pop stack)))
	(push (func var1 var2) stack)))

(let loop ((input (read)) (stack '()))
  (cond
   ((number? input) (loop (read) (push input stack)))
   ((eq? '+ input) (loop (read) (rpn-func + stack)))
   ((eq? '- input) (loop (read) (rpn-func - stack)))
   ((eq? '* input) (loop (read) (rpn-func * stack)))
   ((eq? '/ input) (loop (read) (rpn-func / stack)))
   ((eq? '% input) (loop (read) (rpn-func modulo stack)))
   ((eq? '! input) (loop (read) 
						 (let-values (((var stack) (pop stack))) 
						   (push (fact var) stack))))
   ((eq? '$ input) (loop (read)
						 (let-values (((var stack) (pop stack)))
						   (display var)
						   (newline)
						   stack)))
   ((eq? 'dup input) (loop (read) (dup stack)))
   (else (begin
		   (display "ERROR not valid input: ")
		   (display input)
		   (newline)
		   (loop (read) stack)))))