@staticmethod
    def _start_key(expr):
        """Return start (if possible) else S.Infinity.

        adapted from Set._infimum_key
        """
        try:
            start = expr.start
        except (NotImplementedError,
                AttributeError, __HOLE__):
            start = S.Infinity
        return start
dataset/ETHPy150Open sympy/sympy/sympy/series/sequences.py/SeqBase._start_key