return nullableSynchronizedEntry(delegate().lowerEntry(key), mutex);
      }
    }

    @Override
    @CheckForNull
    public K lowerKey(K key) {
      synchronized (mutex) {
        return delegate().lowerKey(key);
      }
    }

    @Override
    public Set<K> keySet() {
      return navigableKeySet();
    }

    assertEquals("one", map.ceilingKey("one"));
    assertEquals(mapEntry("three", 5), map.higherEntry("one"));
    assertEquals(mapEntry("one", 3), map.ceilingEntry("one"));
    assertEquals("one", map.lowerKey("three"));
    assertEquals("one", map.lowerKey("r"));
    assertEquals("one", map.floorKey("r"));
    assertEquals("three", map.floorKey("three"));

    assertThat(map.descendingMap().entrySet())

    assertEquals("one", map.ceilingKey("one"));
    assertEquals(mapEntry("three", 5), map.higherEntry("one"));
    assertEquals(mapEntry("one", 3), map.ceilingEntry("one"));
    assertEquals("one", map.lowerKey("three"));
    assertEquals("one", map.lowerKey("r"));
    assertEquals("one", map.floorKey("r"));
    assertEquals("three", map.floorKey("three"));

    assertThat(map.descendingMap().entrySet())

  }

  @Override
  @CheckForNull
  public Entry<K, V> lowerEntry(K key) {
    return headMap(key, false).lastEntry();
  }

  @Override
  @CheckForNull
  public K lowerKey(K key) {
    return keyOrNull(lowerEntry(key));
  }

  @Override
  @CheckForNull
  public Entry<K, V> floorEntry(K key) {
    return headMap(key, true).lastEntry();
  }

  @Override

  }

  @Override
  @CheckForNull
  public Entry<K, V> lowerEntry(K key) {
    return headMap(key, false).lastEntry();
  }

  @Override
  @CheckForNull
  public K lowerKey(K key) {
    return keyOrNull(lowerEntry(key));
  }

  @Override
  @CheckForNull
  public Entry<K, V> floorEntry(K key) {
    return headMap(key, true).lastEntry();
  }

  @Override