struct z { };
struct s<T> { };

concept Add<X,Y> { type result; };

model <Y> Add<z,Y> { type result = Y; };

model <X,Y> where { Add<X,Y> } 
Add<s<X>, Y> { 
  type result = s<Add<X,Y>.result>;
};

fun foo(Add<s<z>,s<z> >.result) { }

fun main() -> int@ {
  let x = @s< s<z> >();
  foo(x);
  return 0;
}