M extends Multimap<K, V>>
      Collector<T, ?, M> flatteningToMultimap(
          java.util.function.Function<? super T, ? extends K> keyFunction,
          java.util.function.Function<? super T, ? extends Stream<? extends V>> valueFunction,
          java.util.function.Supplier<M> multimapSupplier) {
    return CollectCollectors.<T, K, V, M>flatteningToMultimap(
        keyFunction, valueFunction, multimapSupplier);
  }