code: @Beta
    protected SortedSet<E> standardSubSet(E fromElement, E toElement) {
      return tailSet(fromElement).headSet(toElement);
    }
docs: A sensible default implementation of {@link #subSet(Object, Object)} in terms of {@link
  #headSet(Object)} and {@link #tailSet(Object)}. In some situations, you may wish to override
  {@link #subSet(Object, Object)} to forward to this implementation.
  
  @since 7.0