def fermat_potential(self, x_image, y_image, x_source, y_source, kwargs_lens):
        """
        fermat potential (negative sign means earlier arrival time)

        :param x_image: image position
        :param y_image: image position
        :param x_source: source position
        :param y_source: source position
        :param kwargs_lens: list of keyword arguments of lens model parameters matching the lens model classes
        :return: fermat potential in arcsec**2 without geometry term (second part of Eqn 1 in Suyu et al. 2013) as a list
        """
        if hasattr(self.lens_model, 'fermat_potential'):
            return self.lens_model.fermat_potential(x_image, y_image, x_source, y_source, kwargs_lens)
        else:
            raise ValueError("Fermat potential is not defined in multi-plane lensing. Please use single plane lens models.")
def fermat_potential ( self , x_image , y_image , x_source , y_source , kwargs_lens ) : if hasattr ( self . lens_model , 'fermat_potential' ) : return self . lens_model . fermat_potential ( x_image , y_image , x_source , y_source , kwargs_lens ) else : raise ValueError ( "Fermat potential is not defined in multi-plane lensing. Please use single plane lens models." )
fermat potential (negative sign means earlier arrival time)

        :param x_image: image position
        :param y_image: image position
        :param x_source: source position
        :param y_source: source position
        :param kwargs_lens: list of keyword arguments of lens model parameters matching the lens model classes
        :return: fermat potential in arcsec**2 without geometry term (second part of Eqn 1 in Suyu et al. 2013) as a list