@property
    def variables(self):
        return tuple(flatten([a.variables for a in self.args]))
simple bincount version, again
dataset/ETHPy150Open sympy/sympy/sympy/series/sequences.py/SeqExprOp.variables--crossover with--dataset/ETHPy150Open statsmodels/statsmodels/statsmodels/tools/grouputils.py/group_sums