def METHOD_NAME(self, graph):
    """ Yield additional ObserverGraph for adding/removing notifiers when
    this observer is encountered in a given ObserverGraph.
    If an observer needs support from another observer(s), e.g.
    for observing 'trait_added' event, then this method can yield any
    number of ObserverGraph containing those additional observer(s).
    If an observer does not need additional support from other observers,
    this method can yield nothing.
    Parameters
    ----------
    graph : ObserverGraph
        The graph where this observer is the root node.
    Yields
    ------
    graph : ObserverGraph
    """
    raise NotImplementedError("iter_extra_graphs must be implemented.")