code: def _add_deflection(self, x, y, alpha_x, alpha_y, kwargs_lens, idex):
          
          theta_x, theta_y = self._co_moving2angle(x, y, idex)
          alpha_x_red, alpha_y_red = self._lens_model.alpha(theta_x, theta_y, kwargs_lens, k=self._sorted_redshift_index[idex])
          alpha_x_phys = self._reduced2physical_deflection(alpha_x_red, idex)
          alpha_y_phys = self._reduced2physical_deflection(alpha_y_red, idex)
          alpha_x_new = alpha_x - alpha_x_phys
          alpha_y_new = alpha_y - alpha_y_phys
          return alpha_x_new, alpha_y_new
docs: adds the pyhsical deflection angle of a single lens plane to the deflection field
  
          :param x: co-moving distance at the deflector plane
          :param y: co-moving distance at the deflector plane
          :param alpha_x: physical angle (radian) before the deflector plane
          :param alpha_y: physical angle (radian) before the deflector plane
          :param kwargs_lens: lens model parameter kwargs
          :param idex: index of the lens model to be added
          :param idex_lens: redshift of the deflector plane
          :return: updated physical deflection after deflector plane (in a backwards ray-tracing perspective)