docs: private ParsingState.Action getAction(int l, int r, int[] heads) {
  		if (heads[l] == r && modifierNumOf(l, heads) == 0)
  			return ParsingState.Action.RIGHT;
  		if (heads[r] == l && modifierNumOf(r, heads) == 0)
  			return ParsingState.Action.LEFT;
  		return ParsingState.Action.SHIFT;
  	}