}

  @Override
  public boolean containsAll(Collection<?> c) {
    return delegate.containsAll(c);
  }

  @Override
  public Iterator<E> descendingIterator() {
    return delegate.descendingIterator();
  }

  @Override
  public NavigableSet<E> descendingSet() {
    return new SafeTreeSet<>(delegate.descendingSet());
  }

  @Override
  public E first() {

  }

  @Override
  public boolean containsAll(Collection<?> c) {
    return delegate.containsAll(c);
  }

  @Override
  public Iterator<E> descendingIterator() {
    return delegate.descendingIterator();
  }

  @Override
  public NavigableSet<E> descendingSet() {
    return new SafeTreeSet<>(delegate.descendingSet());
  }

  @Override
  public E first() {

        return equalsOrThrow(previous, last) ? null : domain.next(previous);
      }
    };
  }

  @GwtIncompatible // NavigableSet
  @Override
  public UnmodifiableIterator<C> descendingIterator() {
    return new AbstractSequentialIterator<C>(last()) {
      final C first = first();

      @Override
      @CheckForNull
      protected C computeNext(C previous) {

        return equalsOrThrow(previous, last) ? null : domain.next(previous);
      }
    };
  }

  @GwtIncompatible // NavigableSet
  @Override
  public UnmodifiableIterator<C> descendingIterator() {
    return new AbstractSequentialIterator<C>(last()) {
      final C first = first();

      @Override
      @CheckForNull
      protected C computeNext(C previous) {

  }

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

  @GwtIncompatible // NavigableSet
  @Override
  public UnmodifiableIterator<E> descendingIterator() {
    return elements.reverse().iterator();
  }

  @Override
  public Spliterator<E> spliterator() {
    return asList().spliterator();
  }

  @Override

  }

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

  @GwtIncompatible // NavigableSet
  @Override
  public UnmodifiableIterator<E> descendingIterator() {
    return elements.reverse().iterator();
  }

  @Override
  public int size() {
    return elements.size();
  }

  @Override
  public boolean contains(@CheckForNull Object o) {
    try {