}

    @Override
    SortedSet<K> backingSet() {
      return (SortedSet<K>) super.backingSet();
    }

    @Override
    public @Nullable Comparator<? super K> comparator() {
      return backingSet().comparator();
    }

    @Override
    public Set<K> keySet() {
      return removeOnlySortedSet(backingSet());
    }

    @Override