docs: def watch_zone(self, zone_id):
          """ Add a zone to the watchlist.
          Zones on the watchlist will push all
          state changes (and those of the source they are currently connected to)
          back to the client """
          r = yield from self._send_cmd(
                  "WATCH %s ON" % (zone_id.device_str(), ))
          self._watched_zones.add(zone_id)
          return r