* the time this method is called.
   */
  Iterator<Subscriber> getSubscribers(Object event) {
    ImmutableSet<Class<?>> eventTypes = flattenHierarchy(event.getClass());

    List<Iterator<Subscriber>> subscriberIterators =
        Lists.newArrayListWithCapacity(eventTypes.size());

    for (Class<?> eventType : eventTypes) {

            HierarchyFixtureInterface.class,
            HierarchyFixtureSubinterface.class,
            HierarchyFixtureParent.class,
            HierarchyFixture.class),
        SubscriberRegistry.flattenHierarchy(HierarchyFixture.class));
  }

  private interface HierarchyFixtureInterface {
    // Exists only for hierarchy mapping; no members.
  }