}

    @Override
    SortedSet<K> createKeySet() {
      return new SortedKeySet(sortedMap());
    }
  }

  private final class NavigableAsMap extends SortedAsMap implements NavigableMap<K, Collection<V>> {

    NavigableAsMap(NavigableMap<K, Collection<V>> submap) {
      super(submap);
    }

    @Override
    NavigableMap<K, Collection<V>> sortedMap() {