def _g(self, z):
        """Helper function to solve Frank copula.

        This functions encapsulates :math:`g_z = e^{-\\theta z} - 1` used on Frank copulas.

        Argument:
            z: np.ndarray

        Returns:
            np.ndarray
        """
        return np.exp(np.multiply(-self.theta, z)) - 1
Helper function to solve Frank copula.

        This functions encapsulates :math:`g_z = e^{-\\theta z} - 1` used on Frank copulas.

        Argument:
            z: np.ndarray

        Returns:
            np.ndarray