return new CustomSetMultimap<>(map, factory);
  }

  private static final class CustomSetMultimap<
          K extends @Nullable Object, V extends @Nullable Object>
      extends AbstractSetMultimap<K, V> {
    transient Supplier<? extends Set<V>> factory;

    CustomSetMultimap(Map<K, Collection<V>> map, Supplier<? extends Set<V>> factory) {
      super(map);
      this.factory = checkNotNull(factory);