}

    @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(