assertTrue(Thread.holdsLock(mutex));
      return delegate().ceilingEntry(key);
    }

    @Override
    public @Nullable K ceilingKey(K key) {
      assertTrue(Thread.holdsLock(mutex));
      return delegate().ceilingKey(key);
    }

    @Override
    public NavigableSet<K> descendingKeySet() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate().descendingKeySet();

    }
  }

  @Override
  public @Nullable Entry<K, V> ceilingEntry(K key) {
    return delegate.ceilingEntry(checkValid(key));
  }

  @Override
  public @Nullable K ceilingKey(K key) {
    return delegate.ceilingKey(checkValid(key));
  }

  @Override
  public void clear() {
    delegate.clear();
  }

  @Override
  public Comparator<? super K> comparator() {

    }
  }

  @Override
  public @Nullable Entry<K, V> ceilingEntry(K key) {
    return delegate.ceilingEntry(checkValid(key));
  }

  @Override
  public @Nullable K ceilingKey(K key) {
    return delegate.ceilingKey(checkValid(key));
  }

  @Override
  public void clear() {
    delegate.clear();
  }

  @Override
  public Comparator<? super K> comparator() {

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

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

    @Override
    public NavigableSet<K> descendingKeySet() {
      return fromMap().descendingKeySet();
    }

    @Override

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

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

    @Override
    public NavigableSet<K> descendingKeySet() {
      return fromMap().descendingKeySet();
    }

    @Override

    assertThat(map.higherKey("one")).isEqualTo("three");
    assertThat(map.higherKey("r")).isEqualTo("three");
    assertThat(map.ceilingKey("r")).isEqualTo("three");
    assertThat(map.ceilingKey("one")).isEqualTo("one");
    assertEquals(mapEntry("three", 5), map.higherEntry("one"));
    assertEquals(mapEntry("one", 3), map.ceilingEntry("one"));