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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 object_ref(self):
 """Return the reference of the changed object."""

        return ImmutableDict(type=self.type, category_id=self.category_id, event_id=self.event_id,
                             session_id=self.session_id, contrib_id=self.contrib_id, subcontrib_id=self.subcontrib_id)