assertEquals(1, Iterators.size(registry.getSubscribers(new Object())));
    assertEquals(2, Iterators.size(registry.getSubscribers(1)));
  }

  public void testGetSubscribers_returnsImmutableSnapshot() {
    StringSubscriber s1 = new StringSubscriber();
    StringSubscriber s2 = new StringSubscriber();
    ObjectSubscriber o1 = new ObjectSubscriber();