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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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)