SortedKeySet(SortedMap<K, Collection<V>> subMap) {
      super(subMap);
    }

    SortedMap<K, Collection<V>> sortedMap() {
      return (SortedMap<K, Collection<V>>) super.map();
    }

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

    @Override

    SortedKeySet(SortedMap<K, Collection<V>> subMap) {
      super(subMap);
    }

    SortedMap<K, Collection<V>> sortedMap() {
      return (SortedMap<K, Collection<V>>) super.map();
    }

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

    @Override