}

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

  @CollectionSize.Require(SEVERAL)
  public void testCeilingHole() {
    resetWithHole();
    assertEquals(a, navigableSet.ceiling(a));

  }

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

  @CollectionSize.Require(SEVERAL)
  public void testCeilingHole() {
    resetWithHole();
    assertEquals(a, navigableSet.ceiling(a));

        NavigableSet<Integer> unfiltered = createUnfiltered(filtered);
        for (int i = 0; i < 10; i++) {
          assertEquals(unfiltered.lower(i), filtered.lower(i));
          assertEquals(unfiltered.floor(i), filtered.floor(i));
          assertEquals(unfiltered.ceiling(i), filtered.ceiling(i));
          assertEquals(unfiltered.higher(i), filtered.higher(i));
        }
      }
    }
  }

    assertThat(set.floor("f")).isNull();
  }

  public void testFloor_elementPresent() {
    ImmutableSortedSet<String> set =
        ImmutableSortedSet.copyOf(new String[] {"e", "a", "e", "f", "b", "i", "d", "a", "c", "k"});
    assertThat(set.floor("f")).isEqualTo("f");
    assertThat(set.floor("j")).isEqualTo("i");
    assertThat(set.floor("q")).isEqualTo("k");
  }

          descendingSet = dS;
          return dS;
        }
        return descendingSet;
      }
    }

    @Override
    @CheckForNull
    public E floor(E e) {
      synchronized (mutex) {
        return delegate().floor(e);
      }
    }

    @Override
    public NavigableSet<E> headSet(E toElement, boolean inclusive) {
      synchronized (mutex) {

          descendingSet = dS;
          return dS;
        }
        return descendingSet;
      }
    }

    @Override
    @CheckForNull
    public E floor(E e) {
      synchronized (mutex) {
        return delegate().floor(e);
      }
    }

    @Override
    public NavigableSet<E> headSet(E toElement, boolean inclusive) {
      synchronized (mutex) {

  @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) {

  @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) {