interface ISemigroup<T>
{
    T Combine(T left, T right);
}

struct IntSumSemigroup : ISemigroup<int> 
{
   public int Combine(int left, int right) => left + right;
}

public T ThreeTimes<T, TSemigroup>(T one) where TSemigroup: struct, ISemigroup<T> {
   var witness = default(TSemigroup);
   return witness.Combine(witness.Combine(one, one), one);
}

using namespace std;
 
template <typename T, typename T2>
decltype(auto) nth_element(T& container, T2 index) {
     return container[index];
}
 
int main() {
   vector<int> v(10,0);
   nth_element(v,1) = 10;
   auto x = nth_element(v,1);
 
   for (auto &i:v) cout << i << '\n';
   cout << "x == " << x << '\n';
}

auto x = nth_element(v,1);

auto x = nth_element(vector<int>{1,2,3},1);