*     key
   * @throws IllegalArgumentException if {@code map} is not empty
   */
  public static <K extends @Nullable Object, V extends @Nullable Object> Multimap<K, V> newMultimap(
      Map<K, Collection<V>> map, Supplier<? extends Collection<V>> factory) {
    return new CustomMultimap<>(map, factory);
  }

   *     key
   * @throws IllegalArgumentException if {@code map} is not empty
   */
  public static <K extends @Nullable Object, V extends @Nullable Object> Multimap<K, V> newMultimap(
      Map<K, Collection<V>> map, Supplier<? extends Collection<V>> factory) {
    return new CustomMultimap<>(map, factory);
  }