def fdiff(self, argindex=4):
        if argindex == 1:
            # Diff wrt n
            raise ArgumentIndexError(self, argindex)
        elif argindex == 2:
            # Diff wrt m
            raise ArgumentIndexError(self, argindex)
        elif argindex == 3:
            # Diff wrt theta
            n, m, theta, phi = self.args
            return (m * cot(theta) * Ynm(n, m, theta, phi) +
                    sqrt((m - n)*(n + m + 1)) * exp(-I*phi) * Ynm(n, m + 1, theta, phi))
        elif argindex == 4:
            # Diff wrt phi
            n, m, theta, phi = self.args
            return I * m * Ynm(n, m, theta, phi)
        else:
            raise ArgumentIndexError(self, argindex)
dataset/ETHPy150Open sympy/sympy/sympy/functions/special/spherical_harmonics.py/Ynm.fdiff/swapped operands of `-`