Derivative of the inverse of the negative binomial transform

        Parameters
        -----------
        z : array-like
            Usually the linear predictor for a GLM or GEE model

        Returns
        -------
        g^(-1)'(z) : array
            The value of the derivative of the inverse of the negative
            binomial link

def inverse_deriv(self, z):
        '''
        Derivative of the inverse of the negative binomial transform

        Parameters
        -----------
        z : array-like
            Usually the linear predictor for a GLM or GEE model

        Returns
        -------
        g^(-1)'(z) : array
            The value of the derivative of the inverse of the negative
            binomial link
        '''
        t = np.exp(z)
        return t / (self.alpha * (1-t)**2)