code: def get_llur(self):
          
          points = np.array([obj.get_llur() for obj in self.objects])
          t_ = points.T
          x1, y1 = t_[0].min(), t_[1].min()
          x2, y2 = t_[2].max(), t_[3].max()
          return (x1, y1, x2, y2)
docs: Get lower-left and upper-right coordinates of the bounding box
          of this compound object.
  
          Returns
          -------
          x1, y1, x2, y2: a 4-tuple of the lower-left and upper-right coords