@Override
  @CheckForNull
  public E lower(E e) {
    return Iterators.<@Nullable E>getNext(headSet(e, false).descendingIterator(), null);
  }

  /** @since 12.0 */
  @Override
  @CheckForNull
  public E floor(E e) {
    return Iterators.<@Nullable E>getNext(headSet(e, true).descendingIterator(), null);
  }

  /** @since 12.0 */
  @Override
  @CheckForNull
  public E ceiling(E e) {

  @Override
  @CheckForNull
  public E lower(E e) {
    return Iterators.<@Nullable E>getNext(headSet(e, false).descendingIterator(), null);
  }

  /** @since 12.0 */
  @Override
  @CheckForNull
  public E floor(E e) {
    return Iterators.<@Nullable E>getNext(headSet(e, true).descendingIterator(), null);
  }

  /** @since 12.0 */
  @Override
  @CheckForNull
  public E ceiling(E e) {

            }
          }
          return elemItr.next();
        }
      };
    }

    @Override
    @GwtIncompatible("NavigableSet")
    public UnmodifiableIterator<C> descendingIterator() {
      return new AbstractIterator<C>() {
        final Iterator<Range<C>> rangeItr = ranges.reverse().iterator();
        Iterator<C> elemItr = emptyIterator();

        @Override
        @CheckForNull

      iterator.remove();
      fail("UnsupportedOperationException expected");
    } catch (UnsupportedOperationException expected) {
    }
    try {
      Iterator<Integer> iterator = unmod.descendingIterator();
      iterator.next();
      iterator.remove();
      fail("UnsupportedOperationException expected");
    } catch (UnsupportedOperationException expected) {
    }
  }