def METHOD_NAME(self, graph, handler, target, dispatcher):
    """ Return a notifier for maintaining downstream observers when
    a set is mutated.
    Parameters
    ----------
    graph : ObserverGraph
        Description for the *downstream* observers, i.e. excluding self.
    handler : callable
        User handler.
    target : object
        Object seen by the user as the owner of the observer.
    dispatcher : callable
        Callable for dispatching the handler.
    Returns
    -------
    notifier : ObserverChangeNotifier
    """
    return ObserverChangeNotifier(
        observer_handler=_observer_change_handler,
        event_factory=set_event_factory,
        prevent_event=lambda event: False,
        graph=graph,
        handler=handler,
        target=target,
        dispatcher=dispatcher,
    )