sclass VarWithNotify<A> extends Var<A> {
  event fireChange;
  
  // clever way (I hope) to do eq() outside of synchronization block
  public void set(A a) {
    A v = get();
    if (eq(v, a)) {
      print ifdef VarWithNotify_debug("VarWithNotify.set no change " + a);
      ret;
    }
    
    print ifdef VarWithNotify_debug("VarWithNotify.set change " + a);
    synchronized { this.v = a; }
    fireChange();
  }
}