code: def external_lensing_effect(self, kwargs_lens, lens_model_internal_bool=None):
          
          alpha0_x, alpha0_y = 0, 0
          kappa_ext = 0
          shear1, shear2 = 0, 0
          if lens_model_internal_bool is None:
              lens_model_internal_bool = [True] * len(kwargs_lens)
          for i, kwargs in enumerate(kwargs_lens):
              if not lens_model_internal_bool[i] is True:
                  f_x, f_y = self._lensModel.alpha(0, 0, kwargs_lens, k=i)
                  f_xx, f_xy, f_yx, f_yy = self._lensModel.hessian(0, 0, kwargs_lens, k=i)
                  alpha0_x += f_x
                  alpha0_y += f_y
                  kappa_ext += (f_xx + f_yy)/2.
                  shear1 += 1./2 * (f_xx - f_yy)
                  shear2 += f_xy
          return alpha0_x, alpha0_y, kappa_ext, shear1, shear2
docs: computes deflection, shear and convergence at (0,0) for those part of the lens model not included in the main deflector
  
          :param kwargs_lens:
          :return: