Ternary operator with std::index_sequence

In the following code:

#include <iostream>
#include <utility>
#include <vector>
#include <set>

template <typename... Args>
void f(Args... args) {
    std::cout << sizeof...(Args) << " elements.\n";
    const std::vector<int> v = {(int)args...};
    for (int x : v) std::cout << x << ' ';   std::cout << '\n';
}

template <std::size_t... Is>
void fhelper (std::index_sequence<Is...>, const std::set<int>& set) {
    f((set.find(Is) == set.end() ? Is : 2*Is)...);
}

int main() {
    fhelper (std::make_index_sequence<10>{}, {1,3,7,8});
}

I want f((set.find(Is) == set.end() ? Is : 2*Is)...); to use Is if set.find(Is) == set.end() but NOTHING otherwise (instead of 2*Is. Thus the number of arguments passes is not fixed. How to achieve that?