}

  @Override
  public UnmodifiableIterator<E> iterator() {
    return forward.descendingIterator();
  }

  @Override
  ImmutableSortedSet<E> headSetImpl(E toElement, boolean inclusive) {
    return forward.tailSet(toElement, inclusive).descendingSet();
  }

  @Override
  ImmutableSortedSet<E> subSetImpl(
      E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) {

  }

  @Override
  public UnmodifiableIterator<E> iterator() {
    return forward.descendingIterator();
  }

  @Override
  ImmutableSortedSet<E> headSetImpl(E toElement, boolean inclusive) {
    return forward.tailSet(toElement, inclusive).descendingSet();
  }

  @Override
  ImmutableSortedSet<E> subSetImpl(
      E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) {

  @Override
  public SortedSet<E> headSet(E toElement) {
    return headSet(toElement, false);
  }

  @Override
  public NavigableSet<E> headSet(E toElement, boolean inclusive) {
    return new SafeTreeSet<>(delegate.headSet(checkValid(toElement), inclusive));
  }

  @Override
  public @Nullable E higher(E e) {
    return delegate.higher(checkValid(e));
  }

  @Override
  public boolean isEmpty() {

  @Override
  public SortedSet<E> headSet(E toElement) {
    return headSet(toElement, false);
  }

  @Override
  public NavigableSet<E> headSet(E toElement, boolean inclusive) {
    return new SafeTreeSet<>(delegate.headSet(checkValid(toElement), inclusive));
  }

  @Override
  public @Nullable E higher(E e) {
    return delegate.higher(checkValid(e));
  }

  @Override
  public boolean isEmpty() {

      for (boolean inclusive : new boolean[] {false, true}) {
        assertThat(navigableMap.headMap(key, inclusive).entrySet())
            .containsExactlyElementsIn(expected.headMap(key, inclusive).entrySet())
            .inOrder();
        assertThat(navigableMap.tailMap(key, inclusive).entrySet())
            .containsExactlyElementsIn(expected.tailMap(key, inclusive).entrySet())
            .inOrder();

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

  @Override
  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

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

  @Override
  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

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

    @Override
    public NavigableMap<K, V> headMap(K toKey, boolean inclusive) {
      assertTrue(Thread.holdsLock(mutex));
      return delegate().headMap(toKey, inclusive);
    }

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

    @Override

    @Override
    public NavigableSet<V> headSet(@ParametricNullness V toElement, boolean inclusive) {
      return wrap(getSortedSetDelegate().headSet(toElement, inclusive));
    }

    @Override
    public NavigableSet<V> tailSet(@ParametricNullness V fromElement, boolean inclusive) {
      return wrap(getSortedSetDelegate().tailSet(fromElement, inclusive));
    }
  }

    ImmutableSortedSet<C> subSet(Range<C> range) {
      return subRangeSet(range).asSet(domain);
    }

    @Override
    ImmutableSortedSet<C> headSetImpl(C toElement, boolean inclusive) {
      return subSet(Range.upTo(toElement, BoundType.forBoolean(inclusive)));
    }

    @Override
    ImmutableSortedSet<C> subSetImpl(
        C fromElement, boolean fromInclusive, C toElement, boolean toInclusive) {