def broadcast_channel(self, message, channel=None):
        """
        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.
        """
        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)
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.