android/guava/src/com/google/common/collect/ForwardingNavigableSet.java
@Beta
  protected NavigableSet<E> standardSubSet(
      E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) {
    return tailSet(fromElement, fromInclusive).headSet(toElement, toInclusive);
  }
@Beta
  protected NavigableSet<E> standardSubSet(
      E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) {
    return tailSet(fromElement, fromInclusive).headSet(toElement, toInclusive);
  }
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.