def METHOD_NAME(event, graph, handler, target, dispatcher):
    """ Maintain downstream notifiers when the trait changes.
    Parameters
    ----------
    event : TraitChangeEvent
        The event that triggers this function.
    graph : ObserverGraph
        Description for the *downstream* observers, i.e. excluding the observer
        that contributed this maintainer function.
    handler : callable
        User handler.
    target : object
        Object seen by the user as the owner of the observer.
    dispatcher : callable
        Callable for dispatching the handler.
    """
    if all(event.old is not skipped for skipped in UNOBSERVABLE_VALUES):
        try:
            add_or_remove_notifiers(
                object=event.old,
                graph=graph,
                handler=handler,
                target=target,
                dispatcher=dispatcher,
                remove=True,
            )
        except NotifierNotFound:
            # The old value could be a default value that does not have any
            # notifier.
            pass
    if all(event.new is not skipped for skipped in UNOBSERVABLE_VALUES):
        add_or_remove_notifiers(
            object=event.new,
            graph=graph,
            handler=handler,
            target=target,
            dispatcher=dispatcher,
            remove=False,
        )