Map<K, Collection<V>> map, Supplier<? extends Collection<V>> factory) {
    return new CustomMultimap<>(map, factory);
  }

  private static final class CustomMultimap<K extends @Nullable Object, V extends @Nullable Object>
      extends AbstractMapBasedMultimap<K, V> {
    transient Supplier<? extends Collection<V>> factory;

    CustomMultimap(Map<K, Collection<V>> map, Supplier<? extends Collection<V>> factory) {
      super(map);