code: def _validate_top_level(self, boxes):
          
          # Add the counts in the superboxes.
          for box in boxes:
              if hasattr(box, 'box'):
                  self._check_superbox_for_top_levels(box.box)
  
          count = self._collect_box_count(boxes)
  
          # If there is one data reference box, then there must also be one ftbl.
          if 'dtbl' in count and 'ftbl' not in count:
              msg = ('The presence of a data reference box requires the '
                     'presence of a fragment table box as well.')
              raise IOError(msg)
docs: Several boxes can only occur at the top level.