// goes well with the stage {} macro (see tok_stages)
srecord Stages(new L</*Stage*/Runnable> stages) is Steppable {
  replace Stage with Runnable.
  int iStage;
  Stage stage;
  
  public bool step() {
    ret iStage <= l(stages) && trueBecause(callF(stage = _get(stages, iStage++)));
  }
  
  void stage(Stage stage) { stages.add(stage); }
}