(defun rpn (stream)
   (let ((stack nil))
     (loop for x = (read stream nil nil)
       while x
       do (if (numberp x)
              (push x stack)
            (let ((a (pop stack))
                  (b (pop stack)))
              (push (funcall x b a) stack)))
       do (format t "~&~a" car stack)
       finally (return stack))))

(rpn *standard-input*)