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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 find(self, source, issuer):
 """ Find a key bundle based on the source of the keys :param source: A source url :param issuer: The issuer of keys :return: A :py:class:`oidcmsg.key_bundle.KeyBundle` instance or None """

        try:
            for kb in self.issuer_keys[issuer]:
                if kb.source == source:
                    return kb
        except KeyError:
            return None

        return None