sclass WithFeedback<A> extends Var<A> {
  S feedback;
  
  *() {}
  *(A value) { super(value); }
  *(S *feedback, A value) { super(value); }
  
  toString {
    if (empty(feedback)) ret super.toString();
    ret super.toString() + "\n" +
      indentx("[Feedback: " + feedback + "]");
  }
}