code: def get_bounding_box(self):
          
          if not isinstance(self.ref_cell, Cell):
              return None
          if (self.rotation is None and self.magnification is None and
                  self.x_reflection is None):
              key = self
          else:
              key = (self.ref_cell, self.rotation, self.magnification,
                     self.x_reflection)
          deps = self.ref_cell.get_dependencies(True)
          if not (self.ref_cell._bb_valid and
                  all(ref._bb_valid for ref in deps) and key in _bounding_boxes):
              for ref in deps:
                  ref.get_bounding_box()
              self.ref_cell.get_bounding_box()
              tmp = self.origin
              self.origin = None
              polygons = self.get_polygons()
              self.origin = tmp
              if len(polygons) == 0:
                  bb = None
              else:
                  all_points = numpy.concatenate(polygons).transpose()
                  bb = numpy.array(((all_points[0].min(), all_points[1].min()),
                                    (all_points[0].max(), all_points[1].max())))
              _bounding_boxes[key] = bb
          else:
              bb = _bounding_boxes[key]
          if self.origin is None or bb is None:
              return bb
          else:
              return bb + numpy.array(((self.origin[0], self.origin[1]),
                                       (self.origin[0], self.origin[1])))
docs: Returns the bounding box for this reference.
  
          Returns
          -------
          out : Numpy array[2,2] or ``None``
              Bounding box of this cell [[x_min, y_min], [x_max, y_max]],
              or ``None`` if the cell is empty.