type t1 {
   int x;
   int y;
}

type t2 <: t1 {
   int add();
}

class c1() : t1 implements t1 {
   init int x;
   init int y;

   export t1 : x, y;
}

subclass c2() : t2 extends c1 at t1 implements t2 with int x, int y {
   init int x;
   init int y;

   super(x = x, y = y);

   int add() { return x + y; }

   export t2: x, y, add;
}

int main() {
  new c2:t2(x = 4, y = 6).add();
}