def pick(self, acr=None):
        """
        Given the authentication context find zero or more authn methods
        that could be used.

        :param acr: The authentication class reference requested
        :return: An URL
        """

        if acr is None:
            # Anything else doesn't make sense
            return self.db.values()
        else:
            return self._pick_by_class_ref(acr)
Given the authentication context find zero or more authn methods
        that could be used.

        :param acr: The authentication class reference requested
        :return: An URL