code: func RouteSubscribeWithOptions(ch chan<- RouteUpdate, done <-chan struct{}, options RouteSubscribeOptions) error {
  	if options.Namespace == nil {
  		none := netns.None()
  		options.Namespace = &none
  	}
  	return routeSubscribeAt(*options.Namespace, netns.None(), ch, done, options.ErrorCallback, options.ListExisting)
  }
docs: // RouteSubscribeWithOptions work like RouteSubscribe but enable to
  // provide additional options to modify the behavior. Currently, the
  // namespace can be provided as well as an error callback.