template <typename ... Args>
void f1(Args... args) {
   std::cout << "f1: ";
   (..., [](auto it) {std::cout << it << " ";}(args));
   std::cout << std::endl;
}
template <typename ... Args>
void f2(Args... args) {
   std::cout << "f2: ";
   (..., [](auto it) {std::cout << it << " ";}(args));
   std::cout << std::endl;
}

template <typename S>
constexpr int find_sep_pos(int) {
   return -1;
}

template <typename S, typename I0, typename ... Tail>
constexpr int find_sep_pos(int init_size) {
   if constexpr (std::is_same_v<I0, S>) {
       return init_size - (sizeof...(Tail) + 1);
   } else {
       return find_sep_pos<S, Tail...>(init_size);
   }
}

template <typename S, typename ... Args>
constexpr int find_sep_pos() {
   return find_sep_pos<S, Args...>(sizeof... (Args));
}

template <int flag, typename Tuple, typename I0, typename ... Tail>
auto first_half_r(Tuple tuple, I0 i0, Tail ... tail) {
   if constexpr (flag > 0) {
       auto new_tup = std::tuple_cat(tuple, std::make_tuple(i0));
    …