*/
  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);
  }

  /**
   * Starts specifying how to combine three {@link ClosingFuture}s into a single pipeline, assuming