MutableEvent history = histories[backOffLevel];
  public Event getHistory(TrainerEvent trainerEvent, int backOffLevel) {    HeadEvent headEvent = (HeadEvent)trainerEvent;    history.clear();    switch (backOffLevel) {    case 0:      // for p(label | w,t)      history.add(headEvent.headWord().word());      history.add(headEvent.headWord().tag());      break;    case 1:      // for p(label | t)      history.add(headEvent.headWord().tag());      break;    }    return history;  }