@Override
    public SortedSet<R> keySet() {
      return (SortedSet<R>) super.keySet();
    }

    @Override
    SortedSet<R> createKeySet() {
      return new Maps.SortedKeySet<>(this);
    }

    @Override
    public @Nullable Comparator<? super R> comparator() {
      return sortedBackingMap().comparator();
    }

    @Override
    public R firstKey() {

    @Override
    public SortedSet<R> keySet() {
      return (SortedSet<R>) super.keySet();
    }

    @Override
    SortedSet<R> createKeySet() {
      return new Maps.SortedKeySet<>(this);
    }

    @Override
    public @Nullable Comparator<? super R> comparator() {
      return sortedBackingMap().comparator();
    }

    @Override
    public R firstKey() {

   * ForwardingSortedMap#keySet} to forward to this implementation or a subclass thereof.
   *
   * @since 15.0
   */
  protected class StandardKeySet extends Maps.SortedKeySet<K, V> {
    /** Constructor for use by subclasses. */
    public StandardKeySet() {
      super(ForwardingSortedMap.this);
    }
  }