code: func (mw *MirrorWriter) broadcastMessage(b []byte) bool {
  	var dropped bool
  	for i, w := range mw.writers {
  		_, err := w.Write(b)
  		if err != nil {
  			mw.writers[i] = nil
  			dropped = true
  		}
  	}
  	return dropped
  }
docs: // broadcastMessage sends the given message to every writer
  // if any writer is killed during the send, 'true' is returned