code: protected E standardHigher(E e) {
      return Iterators.getNext(tailSet(e, false).iterator(), null);
    }
docs: A sensible definition of {@link #higher} in terms of the {@code iterator} method of {@link
  #tailSet(Object, boolean)}. If you override {@link #tailSet(Object, boolean)}, you may wish to
  override {@link #higher} to forward to this implementation.