'L{LogPublisher.addObserver} adds an observer.'

def test_addObserver(self):

    o1 = (lambda e: None)
    o2 = (lambda e: None)
    o3 = (lambda e: None)
    publisher = LogPublisher(o1, o2)
    publisher.addObserver(o3)
    self.assertEqual(set((o1, o2, o3)), set(publisher._observers))
