sclass VarInConcept<A> implements IVar<A> {
  IVar<A> v;
  Concept cc;
  
  *() {} // for persistence
  *(IVar<A> *v, Concept *cc) {}
  
  public void set(A a) { v.set(a); change(); }
  public synchronized A get() { ret v.get(); }
  public synchronized bool has() { ret v.has(); }
  public synchronized void clear() { v.clear(); change(); }
  
  void change() { if (cc != null) cc.change(); }
}