code: def _co_moving2angle_source(self, x, y):
          
          T_z = self._T_z_source
          theta_x = x / T_z
          theta_y = y / T_z
          return theta_x, theta_y
docs: special case of the co_moving2angle definition at the source redshift
  
          :param x:
          :param y:
          :return: