(defun rot13 (ch)
  (cond
    ((<= #\A ch #\Z) (wrap #\A #\Z (+ ch 13)))
    ((<= #\a ch #\z) (wrap #\a #\z (+ ch 13)))
    (t ch)))

   (whilet ((ch (get-char)))
     (put-char (rot13 ch)))