(defun send-simple-response (&key stream code message)
  (write-string code    stream)
  (write-char   #\Space stream)
  (write-string message stream)
  (write-char   #\CR    stream)
  (write-char   #\LF    stream))

(send-simple-response
  :stream  socket-stream
  :code    "501"
  :message "command not recognized")