// only works when owning concept is subclass of ConceptWithChangeListeners
sclass ConceptRefVar<A extends Concept> is IVarWithNotify<A> {
  Concept.Ref<A> ref;
  event fireChange;
  
  *(Concept.Ref<A> *ref) {
    ((ConceptWithChangeListeners) ref.concept()).onChange(l0 fireChange);
  }
  
  public void set(A a) { ref.set(a); }
  public A get() { ret ref!; }
}