code: async def add_listener(self, channel, callback):
          
          self._check_open()
          if channel not in self._listeners:
              await self.fetch('LISTEN {}'.format(utils._quote_ident(channel)))
              self._listeners[channel] = set()
          self._listeners[channel].add(callback)
docs: Add a listener for Postgres notifications.
  
          :param str channel: Channel to listen on.
  
          :param callable callback:
              A callable receiving the following arguments:
              **connection**: a Connection the callback is registered with;
              **pid**: PID of the Postgres server that sent the notification;
              **channel**: name of the channel the notification was sent to;
              **payload**: the payload.