(defun rpn-2 (stream &optional stack)
   (let ((x (read stream nil nil)))
     (when x
       (if (numberp x)
           (push x stack)
         (let ((a (pop stack))
               (b (pop stack)))
           (push (funcall x b a) stack))))
     (format t "~&~a" stack)
     (if x
         (rpn-2 stream stack)
       stack)))

(rpn-2 *standard-input*)