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)
Subscribe to given channel.