type t {
  void m();
}

 class A():t { // no implements necessary
   t state = null;
   void m() {
     state = this;  // do we need coercions here?
   }
   export t : m;
 }

type s <: t { }

 subclass B():s extends A at t {
    super();
    // use of m somewhere
    void m2 { m(); }
    // .: the type of this is s
    // .: the assignment of state = this still works
    //    because s is below t
    export s : m2 as m;
 }