def METHOD_NAME(self, issuer):
    """Fetch client via "iss" in assertion claims. Developers MUST
    implement this method in subclass, e.g.::
        def resolve_issuer_client(self, issuer):
            return Client.query_by_iss(issuer)
    :param issuer: "iss" value in assertion
    :return: Client instance
    """
    raise NotImplementedError()