sclass OrError<A> extends Var<A> {
  O error; // usually a Throwable
  
  *() {}
  *(A value) { super(value); }
  *(bool dummy, O *error) { assertNotNull(error); }
  
  bool ok() { ret error == null; }
  
  toString {
    ret ok()
      ? super.toString()
      : str(error);
  }
}