*/
  public static final class Combiner2<V1 extends @Nullable Object, V2 extends @Nullable Object>
      extends Combiner {

    /**
     * A function that returns a value when applied to the values of the two futures passed to
     * {@link #whenAllSucceed(ClosingFuture, ClosingFuture)}.
     *
     * @param <V1> the type returned by the first future