code: def unsubscribe(self, channel):
          
          try:
              CHANNELS[channel].remove(self.socket.session.session_id)
              self.channels.remove(channel)
          except ValueError:
              return False
          return True
docs: 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.