code: 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.")
docs: 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