docs: public static <T1, T2, R> Maybe<R> combine(Maybe<? extends T1> maybe, Maybe<? extends T2> app,
              BiFunction<? super T1, ? super T2, ? extends R> fn) {
          return narrow(Single.fromPublisher(Future.fromPublisher(maybe.toFlowable())
                                  .zip(Future.fromPublisher(app.toFlowable()), fn)).toMaybe());
      }