* 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() {