flatteningToMultimap(
            input -> checkNotNull(keyFunction.apply(input)),
            input -> valuesFunction.apply(input).peek(Preconditions::checkNotNull),
            MultimapBuilder.linkedHashKeys().arrayListValues()::<K, V>build),
        ImmutableListMultimap::copyOf);
  }

  static <T extends @Nullable Object, K, V>
      Collector<T, ?, ImmutableSetMultimap<K, V>> toImmutableSetMultimap(

   * back into the multimap, that key will come last in the key iteration order.
   */
  public static MultimapBuilderWithKeys<@Nullable Object> linkedHashKeys() {
    return linkedHashKeys(DEFAULT_EXPECTED_KEYS);
  }

  /**
   * Uses an hash table to map keys to value collections, initialized to expect the specified number
   * of keys.
   *