docs: async def _subscribe(self, channels, is_mask):
          """Subscribe to given channel."""
          news = []
          for channel in channels:
              key = channel, is_mask
              self._channels.append(key)
              if key in self._plugin._subscriptions:
                  self._plugin._subscriptions[key].append(self._queue)
              else:
                  self._plugin._subscriptions[key] = [self._queue]
                  news.append(channel)
          if news:
              await getattr(self._sub, 'psubscribe' if is_mask else 'subscribe')(news)