*
     * @param <V1> the type returned by the first future
     * @param <V2> the type returned by the second future
     * @param <U> the type returned by the function
     */
    public interface AsyncClosingFunction2<
        V1 extends @Nullable Object, V2 extends @Nullable Object, U extends @Nullable Object> {

      /**
       * Applies this function to two inputs, or throws an exception if unable to do so.
       *