code: @Beta
    protected NavigableSet<E> standardSubSet(
        E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) {
      return tailSet(fromElement, fromInclusive).headSet(toElement, toInclusive);
    }
docs: A sensible definition of {@link #subSet(Object, boolean, Object, boolean)} in terms of the
  {@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.