code: def _validate_singletons(self, boxes):
          
          count = self._collect_box_count(boxes)
          # Which boxes occur more than once?
          multiples = [box_id for box_id, bcount in count.items() if bcount > 1]
          if 'dtbl' in multiples:
              raise IOError('There can only be one dtbl box in a file.')
docs: Several boxes can only occur once.