def unsubscribe(self, channel):
        """
        Remove the channel from this socket's channels, and from the
        list of subscribed session IDs for the channel. Return False
        if not subscribed, otherwise True.
        """
        try:
            CHANNELS[channel].remove(self.socket.session.session_id)
            self.channels.remove(channel)
        except ValueError:
            return False
        return True
Remove the channel from this socket's channels, and from the
        list of subscribed session IDs for the channel. Return False
        if not subscribed, otherwise True.