@property
    def header(self):
        if not self._ast:
            return None
        else:
            return self._ast[0]
Calculate the r frontier where we must switch from Nr to Sr
dataset/ETHPy150Open xtrementl/focus/focus/parser/parser.py/SettingParser.header--crossover with--dataset/ETHPy150Open nltk/nltk/nltk/probability.py/SimpleGoodTuringProbDist._switch