def fdiff(self, argindex=1):
        from sympy import polygamma
        if argindex == 1:
            # http://functions.wolfram.com/GammaBetaErf/Binomial/20/01/01/
            n, k = self.args
            return binomial(n, k)*(polygamma(0, n + 1) - \
                polygamma(0, n - k + 1))
        elif argindex == 2:
            # http://functions.wolfram.com/GammaBetaErf/Binomial/20/01/02/
            n, k = self.args
            return binomial(n, k)*(polygamma(0, k + 1 - n) - \
                polygamma(0, k + 1))
        else:
            raise ArgumentIndexError(self, argindex)
dataset/ETHPy150Open sympy/sympy/sympy/functions/combinatorial/factorials.py/binomial.fdiff/swapped operands of `-`