public @Nullable Entry<K, V2> floorEntry(@ParametricNullness K key) {
      return transformEntry(fromMap().floorEntry(key));
    }

    @Override
    public @Nullable K floorKey(@ParametricNullness K key) {
      return fromMap().floorKey(key);
    }

    @Override
    public NavigableMap<K, V2> headMap(@ParametricNullness K toKey) {
      return headMap(toKey, false);
    }

    @Override

        if (subRange.isEmpty()) {
          return emptyIterator();
        }
        Cut<K> cutToStart =
            MoreObjects.firstNonNull(
                entriesByLowerBound.floorKey(subRange.lowerBound), subRange.lowerBound);
        Iterator<RangeMapEntry<K, V>> backingItr =
            entriesByLowerBound.tailMap(cutToStart, true).values().iterator();

    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())
        .containsExactly(mapEntry("two", 3), mapEntry("three", 5), mapEntry("one", 3))
        .inOrder();

        if (subRange.isEmpty()) {
          return emptyIterator();
        }
        Cut<K> cutToStart =
            MoreObjects.firstNonNull(
                entriesByLowerBound.floorKey(subRange.lowerBound), subRange.lowerBound);
        Iterator<RangeMapEntry<K, V>> backingItr =
            entriesByLowerBound.tailMap(cutToStart, true).values().iterator();

      synchronized (mutex) {
        return nullableSynchronizedEntry(delegate().floorEntry(key), mutex);
      }
    }

    @Override
    public @Nullable K floorKey(K key) {
      synchronized (mutex) {
        return delegate().floorKey(key);
      }
    }

    @Override
    public NavigableMap<K, V> headMap(K toKey, boolean inclusive) {
      synchronized (mutex) {

      synchronized (mutex) {
        return nullableSynchronizedEntry(delegate().floorEntry(key), mutex);
      }
    }

    @Override
    public @Nullable K floorKey(K key) {
      synchronized (mutex) {
        return delegate().floorKey(key);
      }
    }

    @Override
    public NavigableMap<K, V> headMap(K toKey, boolean inclusive) {
      synchronized (mutex) {

    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())
        .containsExactly(mapEntry("two", 3), mapEntry("three", 5), mapEntry("one", 3))
        .inOrder();

    return keyOrNull(lowerEntry(key));
  }

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

  @Override
  public @Nullable K floorKey(K key) {
    return keyOrNull(floorEntry(key));
  }

  @Override
  public @Nullable Entry<K, V> ceilingEntry(K key) {
    return tailMap(key, true).firstEntry();
  }

  @Override