(defun stringp (obj)
 (eq (type-of obj) 'string))

(defun greet-with (message &key stream)
 (native-write message :stream stream))

(defun be-sad ()
 (native-write "Ok, bye then\n"))

(if (stringp *msg*)
 (progn
  (greet-with *msg*))
 (be-sad))