def METHOD_NAME(self) -> None:
    """
    L{LogPublisher.removeObserver} removes an observer.
    """
    o1 = cast(ILogObserver, lambda e: None)
    o2 = cast(ILogObserver, lambda e: None)
    o3 = cast(ILogObserver, lambda e: None)
    publisher = LogPublisher(o1, o2, o3)
    publisher.removeObserver(o2)
    self.assertEqual({o1, o3}, set(publisher._observers))