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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 get_issuer_keys(self, issuer):
 """ Get all the keys that belong to an entity. :param issuer: The entity ID :return: A possibly empty list of keys """

        res = []
        for kbl in self.issuer_keys[issuer]:
            res.extend(kbl.keys())
        return res