android/guava/src/com/google/common/collect/ForwardingNavigableSet.java
protected E standardCeiling(E e) {
    return Iterators.getNext(tailSet(e, true).iterator(), null);
  }
protected E standardCeiling(E e) {
    return Iterators.getNext(tailSet(e, true).iterator(), null);
  }
A sensible definition of {@link #ceiling} 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 #ceiling} to forward to this implementation.