def _r_Nr(self):
        nonzero = self._r_Nr_non_zero()

        if not nonzero:
            return [], []
        return zip(*sorted(nonzero.items()))
Split the frequency distribution in two list (r, Nr), where Nr(r) > 0
dataset/ETHPy150Open nltk/nltk/nltk/probability.py/SimpleGoodTuringProbDist._r_Nr