super(key, delegate, ancestor);
    }
  }

  @Override
  Set<K> createKeySet() {
    return new KeySet(map);
  }

  final Set<K> createMaybeNavigableKeySet() {
    if (map instanceof NavigableMap) {
      return new NavigableKeySet((NavigableMap<K, Collection<V>>) map);
    } else if (map instanceof SortedMap) {