code: def _validate(self, writing=False):
          
          box_ids = [box.box_id for box in self.box]
          if len(box_ids) != 1 or box_ids[0] != 'flst':
              msg = ("Fragment table boxes must have a single fragment list "
                     "box as a child box.")
              self._dispatch_validation_error(msg, writing=writing)
docs: Self-validate the box before writing.