* {@code headSet} and {@code tailSet} methods. In many cases, you may wish to override {@link
   * #subSet(Object, boolean, Object, boolean)} to forward to this implementation.
   */
  protected NavigableSet<E> standardSubSet(
      @ParametricNullness E fromElement,
      boolean fromInclusive,
      @ParametricNullness E toElement,
      boolean toInclusive) {

   * {@code headSet} and {@code tailSet} methods. In many cases, you may wish to override {@link
   * #subSet(Object, boolean, Object, boolean)} to forward to this implementation.
   */
  protected NavigableSet<E> standardSubSet(
      @ParametricNullness E fromElement,
      boolean fromInclusive,
      @ParametricNullness E toElement,
      boolean toInclusive) {

    }

    @Override
    public SortedSet<E> subSet(@ParametricNullness E fromElement, @ParametricNullness E toElement) {
      return standardSubSet(fromElement, toElement);
    }

    @Override
    public NavigableSet<E> headSet(@ParametricNullness E toElement, boolean inclusive) {
      return forward.tailSet(toElement, inclusive).descendingSet();