MutableEvent history = histories[backOffLevel];
  public Event getHistory(TrainerEvent trainerEvent, int backOffLevel) {    HeadEvent headEvent = (HeadEvent)trainerEvent;    Sexp noGapParent =      Language.training.removeGapAugmentation(headEvent.parent());    history.clear();    switch (backOffLevel) {    case 0:      // for p(H | P, w, t)      history.add(noGapParent);      history.add(headEvent.headWord().word());      history.add(headEvent.headWord().tag());      break;    case 1:      // for p(H | P, t)      history.add(noGapParent);      history.add(headEvent.headWord().tag());      break;    case 2:      // for p(H | P)      history.add(noGapParent);    }    return history;  }