def verify(self, user, device=None):
        """Verify that the `user` and optional `device` exist. Returns
        `None` when the user/device does not exist or a list of the user's
        devices otherwise.
        """
        payload = {"user": user, "token": self.token}
        if device:
            payload["device"] = device
        try:
            request = Request("post", USER_URL, payload)
        except RequestError:
            return None
        else:
            return request.answer["devices"]