// primary template
template<typename T, typename A, typename B> struct Replace;
// in this case there is general definition that makes sense
// so just leave it undefined

// Case: empty tuple (this is what needed to be added)
template<typename A, typename B>
struct Replace<tuple<>, A, B> {
  typedef tuple<> Result;
}

// Case: type not found
template<typename H, typename...Ts, typename A, typename B>
struct Replace<tuple<H, Ts...>, A, B> : public Append<tuple<H>, typename Replace<tuple<Ts...>, A, B>::Result> {
  
};

// Case: type found!
template<typename... Ts, typename A, typename B>
struct Replace<tuple<A, Ts...>, A, B> {
  typedef tuple <B, Ts...> Result;
};