*     the arguments, or if any has already been {@linkplain #finishToFuture() finished}
   */
  public static <V1 extends @Nullable Object, V2 extends @Nullable Object>
      Combiner2<V1, V2> whenAllSucceed(ClosingFuture<V1> future1, ClosingFuture<V2> future2) {
    return new Combiner2<>(future1, future2);
  }

  /**

  static <K extends @Nullable Object, V1 extends @Nullable Object, V2 extends @Nullable Object>
      EntryTransformer<K, V1, V2> asEntryTransformer(final Function<? super V1, V2> function) {
    checkNotNull(function);
    return new EntryTransformer<K, V1, V2>() {
      @Override
      @ParametricNullness
      public V2 transformEntry(@ParametricNullness K key, @ParametricNullness V1 value) {