* thereof.
   *
   * @since 12.0
   */
  protected class StandardNavigableKeySet extends Maps.NavigableKeySet<K, V> {
    /** Constructor for use by subclasses. */
    public StandardNavigableKeySet() {
      super(ForwardingNavigableMap.this);
    }
  }

  @Override
  public NavigableSet<K> descendingKeySet() {

   * thereof.
   *
   * @since 12.0
   */
  protected class StandardNavigableKeySet extends Maps.NavigableKeySet<K, V> {
    /** Constructor for use by subclasses. */
    public StandardNavigableKeySet() {
      super(ForwardingNavigableMap.this);
    }
  }

  @Override
  public NavigableSet<K> descendingKeySet() {

    @Override
    public NavigableMap<K, V> descendingMap() {
      return new StandardDescendingMap();
    }

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

    @Override
    public NavigableSet<K> descendingKeySet() {
      return standardDescendingKeySet();
    }

    @Override
    public K firstKey() {