@Override
    public SortedMap<K, V> headMap(K toKey) {
      return headMap(toKey, false);
    }

    @Override
    public @Nullable Entry<K, V> higherEntry(K key) {
      assertTrue(Thread.holdsLock(mutex));
      return delegate().higherEntry(key);
    }

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

  public NavigableMap<K, V> headMap(K toKey, boolean inclusive) {
    return new SafeTreeMap<>(delegate.headMap(checkValid(toKey), inclusive));
  }

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

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

  @Override
  public boolean isEmpty() {

  public NavigableMap<K, V> headMap(K toKey, boolean inclusive) {
    return new SafeTreeMap<>(delegate.headMap(checkValid(toKey), inclusive));
  }

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

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

  @Override
  public boolean isEmpty() {

      return transformEntries(fromMap().headMap(toKey, inclusive), transformer);
    }

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

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

    @Override

      return transformEntries(fromMap().headMap(toKey, inclusive), transformer);
    }

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

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

    @Override

    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"));
    assertThat(map.lowerKey("three")).isEqualTo("one");
    assertThat(map.lowerKey("r")).isEqualTo("one");