code: def sort_arrival_times(self, x_mins, y_mins, kwargs_lens):
          
  
          if hasattr(self.lensModel, '_no_potential'):
              raise Exception('Instance of lensModel passed to this class does not compute the lensing potential, '
                              'and therefore cannot compute time delays.')
  
          if len(x_mins) <= 1:
              return x_mins, y_mins
          x_source, y_source = self.lensModel.ray_shooting(x_mins, y_mins, kwargs_lens)
          x_source = np.mean(x_source)
          y_source = np.mean(y_source)
          if self.lensModel.multi_plane is True:
              arrival_time = self.lensModel.arrival_time(x_mins, y_mins, kwargs_lens)
          else:
              fermat_pot = self.lensModel.fermat_potential(x_mins, y_mins, x_source, y_source, kwargs_lens)
              arrival_time = fermat_pot
          idx = np.argsort(arrival_time)
          x_mins = np.array(x_mins)[idx]
          y_mins = np.array(y_mins)[idx]
          return x_mins, y_mins
docs: sort arrival times (fermat potential) of image positions in increasing order of light travel time
          :param x_mins: ra position of images
          :param y_mins: dec position of images
          :param kwargs_lens: keyword arguments of lens model
          :return: sorted lists of x_mins and y_mins