def METHOD_NAME(self) -> None:
    """
    L{LogPublisher.removeObserver} removes an observer that is not
    registered.
    """
    o1 = cast(ILogObserver, lambda e: None)
    o2 = cast(ILogObserver, lambda e: None)
    o3 = cast(ILogObserver, lambda e: None)
    publisher = LogPublisher(o1, o2)
    publisher.removeObserver(o3)
    self.assertEqual({o1, o2}, set(publisher._observers))