def _eval_derivative(self, x):
        """
        Differentiate wrt x as long as x is not in the free symbols of any of
        the upper or lower limits.

        Sum(a*b*x, (x, 1, a)) can be differentiated wrt x or b but not `a`
        since the value of the sum is discontinuous in `a`. In a case
        involving a limit variable, the unevaluated derivative is returned.
        """

        # diff already confirmed that x is in the free symbols of self, but we
        # don't want to differentiate wrt any free symbol in the upper or lower
        # limits
        # XXX remove this test for free_symbols when the default _eval_derivative is in
        if x not in self.free_symbols:
            return S.Zero

        # get limits and the function
        f, limits = self.function, list(self.limits)

        limit = limits.pop(-1)

        if limits:  # f is the argument to a Sum
            f = self.func(f, *limits)

        if len(limit) == 3:
            _, a, b = limit
            if x in a.free_symbols or x in b.free_symbols:
                return None
            df = Derivative(f, x, evaluate=True)
            rv = self.func(df, limit)
            if limit[0] not in df.free_symbols:
                rv = rv.doit()
            return rv
        else:
            return NotImplementedError('Lower and upper bound expected.')
dataset/ETHPy150Open sympy/sympy/sympy/concrete/summations.py/Sum._eval_derivative/original