Modification to LISTEN

[Glenn S. Burke <GSB@MIT-MC.ARPA>]

Aside from some already mentioned reasons, one can always do

(defun listen (&optional (input-stream *standard-input*))
  (let ((char (read-char-no-hang stream nil nil)))
    (cond ((null char) nil)
	  (t (unread-char char) t))))

instead of the other way around, without changing the definition of
anything.