sclass DoneFlag extends Flag {
  transient volatile O error;
  transient bool printErrors;
  
  *() {}
  *(bool *printErrors) {}
  
  void done() { raise(); }
  void done aka setError(O error) {
    this.error = error;
    if (printErrors)
      printStackTrace_gen(error);
    raise();
  }
  
  bool hasError() { ret error != null; }
  bool isDone() { ret isUp(); }
}