return descendingIterator().next();
  }

  @Override
  public NavigableSet<E> descendingSet() {
    return delegate().descendingSet();
  }

  /**
   * A sensible implementation of {@link NavigableSet#descendingSet} in terms of the other methods
   * of {@link NavigableSet}, notably including {@link NavigableSet#descendingIterator}.
   *

  protected <E> NavigableSet<E> create() {
    TestSet<E> inner =
        new TestSet<>(new TreeSet<E>((Comparator<E>) Ordering.natural().nullsFirst()), null);
    NavigableSet<E> outer = Synchronized.navigableSet(inner, null);
    inner.mutex = outer;
    return outer;
  }

  static class TestSet<E> extends SynchronizedSetTest.TestSet<E> implements NavigableSet<E> {

    TestSet(NavigableSet<E> delegate, @Nullable Object mutex) {

  @SuppressWarnings("unchecked")
  protected <E> NavigableSet<E> create() {
    TestSet<E> inner =
        new TestSet<>(new TreeSet<E>((Comparator<E>) Ordering.natural().nullsFirst()), MUTEX);
    NavigableSet<E> outer = Synchronized.navigableSet(inner, MUTEX);
    return outer;
  }

  static class TestSet<E> extends SynchronizedSetTest.TestSet<E> implements NavigableSet<E> {

    TestSet(NavigableSet<E> delegate, @Nullable Object mutex) {

                return list;
              }

              @Override
              public Set<E> create(Object... elements) {
                NavigableSet<E> navigableSet = (NavigableSet<E>) delegate.create(elements);
                return navigableSet.descendingSet();
              }
            })
        .named(parentBuilder.getName() + " descending")
        .withFeatures(features)

                return list;
              }

              @Override
              public Set<E> create(Object... elements) {
                NavigableSet<E> navigableSet = (NavigableSet<E>) delegate.create(elements);
                return navigableSet.descendingSet();
              }
            })
        .named(parentBuilder.getName() + " descending")
        .withFeatures(features)

    resetWithHole();
    assertEquals(null, navigableSet.lower(a));
    assertEquals(a, navigableSet.lower(b));
    assertEquals(a, navigableSet.lower(c));
  }

  @CollectionSize.Require(SEVERAL)
  public void testFloorHole() {
    resetWithHole();
    assertEquals(a, navigableSet.floor(a));
    assertEquals(a, navigableSet.floor(b));
    assertEquals(c, navigableSet.floor(c));
  }

        NavigableSet<Integer> filtered = filter(createUnfiltered(contents), EVEN);
        NavigableSet<Integer> unfiltered = createUnfiltered(filtered);

        assertEquals(unfiltered.pollLast(), filtered.pollLast());
        assertEquals(unfiltered, filtered);
      }
    }

    public void testNavigation() {
      for (List<Integer> contents : SAMPLE_INPUTS) {

@GwtIncompatible
public final class SafeTreeSet<E> implements Serializable, NavigableSet<E> {
  @SuppressWarnings("unchecked")
  private static final Comparator<Object> NATURAL_ORDER =
      new Comparator<Object>() {
        @Override
        public int compare(Object o1, Object o2) {
          return ((Comparable<Object>) o1).compareTo(o2);
        }
      };

  private final NavigableSet<E> delegate;

  public SafeTreeSet() {

    resetWithHole();
    assertEquals(null, navigableSet.lower(a));
    assertEquals(a, navigableSet.lower(b));
    assertEquals(a, navigableSet.lower(c));
  }

  @CollectionSize.Require(SEVERAL)
  public void testFloorHole() {
    resetWithHole();
    assertEquals(a, navigableSet.floor(a));
    assertEquals(a, navigableSet.floor(b));
    assertEquals(c, navigableSet.floor(c));
  }

    new ForwardingWrapperTester()
        .testForwarding(
            NavigableSet.class,
            new Function<NavigableSet, NavigableSet>() {
              @Override
              public NavigableSet apply(NavigableSet delegate) {
                return wrap(delegate);
              }
            });
  }

  public void testEquals() {
    NavigableSet<String> set1 = ImmutableSortedSet.of("one");