}

    @Override
    NavigableSet<K> createKeySet() {
      return new NavigableKeySet(sortedMap());
    }

    @Override
    public NavigableSet<K> navigableKeySet() {
      return keySet();
    }

    @Override
    public NavigableSet<K> descendingKeySet() {
      return descendingMap().navigableKeySet();
    }

    @Override
    public NavigableMap<K, Collection<V>> subMap(