def _validate(self, writing=False):
        """Self-validate the box before writing."""
        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)