SortedSetMultimap<K, V> newSortedSetMultimap(
          Map<K, Collection<V>> map, Supplier<? extends SortedSet<V>> factory) {
    return new CustomSortedSetMultimap<>(map, factory);
  }

  private static final class CustomSortedSetMultimap<
          K extends @Nullable Object, V extends @Nullable Object>
      extends AbstractSortedSetMultimap<K, V> {
    transient Supplier<? extends SortedSet<V>> factory;