history.add(headEvent.headWord().tag());
  public Event getHistory(TrainerEvent trainerEvent, int backOffLevel) {    HeadEvent headEvent = (HeadEvent)trainerEvent;    history.clear();    switch (backOffLevel) {    case 0:      // for p(w,t | H, +TOP+)      history.add(headEvent.head());      history.add(headEvent.parent());      break;    case 1:      // for p(w,t | +TOP+)      history.add(headEvent.parent());      break;    }    return history;  }