concept Comparable<T> {
  fun better(T,T) -> bool@;
};

struct Apple {
  int rating;
};
model Comparable<Apple> {
  fun better(Apple a, Apple b) -> bool@ {
    return a.rating > b.rating;
  }
};

fun pick<T> where { Comparable<T> }
(T x, T y) -> T {
  if (better(x,y)) 
    return x;
  else
    return y;
}

fun main() -> int@ {
  let a = @Apple{ rating = 1 };
  let b = @Apple{ rating = 2 };
  let c = pick(a,b);
  if (c.rating == 2) return 0;
  else return -1;
}