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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 _setdefault(self, relation):
 """Add a relation to the cache, or return it if it already exists. :param _CachedRelation relation: The relation to set or get. :return _CachedRelation: The relation stored under the given relation's key """

        self.add_schema(relation.database, relation.schema)
        key = relation.key()
        return self.relations.setdefault(key, relation)