*     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);
  }

  /**

      @Override
      @ParametricNullness
      public V2 apply(@ParametricNullness V1 v1) {
        return transformer.transformEntry(key, v1);
      }
    };
  }

  /** Views an entry transformer as a function from {@code Entry} to values. */
  static <K extends @Nullable Object, V1 extends @Nullable Object, V2 extends @Nullable Object>
      Function<Entry<K, V1>, V2> asEntryToValueFunction(