code: def _check_superbox_for_top_levels(self, boxes):
          
          # We are only looking at the boxes contained in a superbox, so if any
          # of the blacklisted boxes show up here, it's an error.
          TOP_LEVEL_ONLY_BOXES = set(['dtbl'])
          box_ids = set([box.box_id for box in boxes])
          intersection = box_ids.intersection(TOP_LEVEL_ONLY_BOXES)
          if len(intersection) > 0:
              msg = "A {0} box cannot be nested in a superbox."
              raise IOError(msg.format(list(intersection)[0]))
  
          # Recursively check any contained superboxes.
          for box in boxes:
              if hasattr(box, 'box'):
                  self._check_superbox_for_top_levels(box.box)
docs: Several boxes can only occur at the top level.