checkNotNull(action);
    map.forEach(
        (key, valueCollection) -> valueCollection.forEach(value -> action.accept(key, value)));
  }

  @Override
  Map<K, Collection<V>> createAsMap() {
    return new AsMap(map);
  }

  final Map<K, Collection<V>> createMaybeNavigableAsMap() {
    if (map instanceof NavigableMap) {
      return new NavigableAsMap((NavigableMap<K, Collection<V>>) map);