@Override
  abstract Set<V> createCollection();

  @Override
  Set<V> createUnmodifiableEmptyCollection() {
    return emptySet();
  }

  @Override
  <E extends @Nullable Object> Collection<E> unmodifiableCollectionSubclass(
      Collection<E> collection) {
    return unmodifiableSet((Set<E>) collection);
  }

  @Override
  Collection<V> wrapCollection(@ParametricNullness K key, Collection<V> collection) {

  @Override
  List<V> createUnmodifiableEmptyCollection() {
    return emptyList();
  }

  @Override
  <E extends @Nullable Object> Collection<E> unmodifiableCollectionSubclass(
      Collection<E> collection) {
    return unmodifiableList((List<E>) collection);
  }

  @Override
  Collection<V> wrapCollection(@ParametricNullness K key, Collection<V> collection) {