docs: public static <T, R1, R> Single<R> forEach(Single<? extends T> value1,
                                               Function<? super T, Single<R1>> value2,
                                               BiFunction<? super T, ? super R1, ? extends R> yieldingFunction) {
  
          Single<R> res = value1.flatMap(in -> {
  
              Single<R1> a = value2.apply(in);
              return a.map(ina -> yieldingFunction.apply(in, ina));
  
  
          });
  
  
          return narrow(res);
  
      }