code: def broadcast_channel(self, message, channel=None):
          
          if channel is None:
              channels = self.channels
          else:
              channels = [channel]
          for channel in channels:
              for subscriber in CHANNELS[channel]:
                  if subscriber != self.socket.session.session_id:
                      session = self.socket.handler.server.sessions[subscriber]
                      self._write(message, session)
docs: Send the given message to all subscribers for the channel
          given. If no channel is given, send to the subscribers for
          all the channels that this socket is subscribed to.