'Derivative of the inverse of the power transform
Parameters
z : array-like
`z` is 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 power transform
function'

def inverse_deriv(self, z):

    if (self.power == 1):
        return np.ones_like(z)
    else:
        return (np.power(z, ((1 - self.power) / self.power)) / self.power)
