code: def verify(self, user, device=None):
          
          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"]
docs: 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.