def __setitem__(self, k, v):
        if v in None:
            raise ValueError('NLMSA cannot save edge-info. Only \
                             nlmsa[s1][s2]=None allowed')
        self += k
dataset/ETHPy150Open cjlee112/pygr/pygr/nlmsa_utils.py/BuildMSASlice.__setitem__/operator `is not` -> `in`