}

    @Nullable SortedSet<K> sortedKeySet;

    // returns a SortedSet, even though returning a Set would be sufficient to
    // satisfy the SortedMap.keySet() interface
    @Override
    public SortedSet<K> keySet() {
      SortedSet<K> result = sortedKeySet;
      return (result == null) ? sortedKeySet = createKeySet() : result;
    }

    @Override

      return new SortedKeySet<>(map().subMap(fromElement, toElement));
    }

    @Override
    public SortedSet<K> headSet(@ParametricNullness K toElement) {
      return new SortedKeySet<>(map().headMap(toElement));
    }

    @Override
    public SortedSet<K> tailSet(@ParametricNullness K fromElement) {
      return new SortedKeySet<>(map().tailMap(fromElement));
    }