template<typename T1, typename T2>
 ??? gcd(T1 a, T2 b){
   static_assert(std::is_integral<T1>::value, "T1 should be an integral type!");
   static_assert(std::is_integral<T2>::value, "T2 should be an integral type!");
   if( b == 0 ){ return a; }
   else{
     return gcd(b, a % b);
   }
 }