}

    @Override
    public NavigableSet<E> subSet(
        @ParametricNullness E fromElement,
        boolean fromInclusive,
        @ParametricNullness E toElement,
        boolean toInclusive) {
      return forward.subSet(toElement, toInclusive, fromElement, fromInclusive).descendingSet();
    }

    @Override
    public SortedSet<E> subSet(@ParametricNullness E fromElement, @ParametricNullness E toElement) {

    }

    @Override
    public NavigableSet<E> subSet(
        E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) {
      synchronized (mutex) {
        return Synchronized.navigableSet(
            delegate().subSet(fromElement, fromInclusive, toElement, toInclusive), mutex);
      }
    }

    @Override
    public SortedSet<E> subSet(E fromElement, E toElement) {

    }

    @Override
    public NavigableSet<E> subSet(
        E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) {
      synchronized (mutex) {
        return Synchronized.navigableSet(
            delegate().subSet(fromElement, fromInclusive, toElement, toInclusive), mutex);
      }
    }

    @Override
    public SortedSet<E> subSet(E fromElement, E toElement) {