android/guava/src/com/google/common/collect/ForwardingNavigableSet.java
protected E standardLower(E e) {
    return Iterators.getNext(headSet(e, false).descendingIterator(), null);
  }
protected E standardLower(E e) {
    return Iterators.getNext(headSet(e, false).descendingIterator(), null);
  }
A sensible definition of {@link #lower} in terms of the {@code descendingIterator} method of
{@link #headSet(Object, boolean)}. If you override {@link #headSet(Object, boolean)}, you may
wish to override {@link #lower} to forward to this implementation.