@staticmethod
def first_order_score(y, mean, scale, shape, skewness):
    """ GAS Exponential Update term using gradient only - native Python function

        Parameters
        ----------
        y : float
            datapoint for the time series

        mean : float
            location parameter for the Exponential distribution

        scale : float
            scale parameter for the Exponential distribution

        shape : float
            tail thickness parameter for the Exponential distribution

        skewness : float
            skewness parameter for the Exponential distribution

        Returns
        ----------
        - Score of the Exponential family
        """
    return 1 - mean * y
