}

    @SuppressWarnings("unchecked") // proxy for a generic class
    <V1, V2, U> ClosingFunction2<V1, V2, U> waitFor(ClosingFunction2<V1, V2, U> closingFunction2) {
      return waitFor(closingFunction2, ClosingFunction2.class);
    }

    @SuppressWarnings("unchecked") // proxy for a generic class
    <V1, V2, U> AsyncClosingFunction2<V1, V2, U> waitFor(

     *
     * @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 ClosingFunction2<
        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.
       *

     * @param <V2> the type returned by the second future
     * @param <U> the type returned by the function
     */
    @FunctionalInterface
    public interface ClosingFunction2<
        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.
       *