docs: def _is_loggedin(self, auth_secret):
          """Check if a user is logged-in by verifying the input authentication secret.
  
          Parameters
          ----------
          auth_secret: str
              The authentication secret of a logged-in user.
  
          Returns
          -------
          bool
              True if the authentication secret is valid, False otherwise.
          userid: str
              The user ID associated with the authentication secret if the authentication secret
              valid, None otherwise.
          """
          # Get the userid from the authentication secret.
          userid = self._rc.hget(pytwis_constants.AUTHS_KEY, auth_secret)
          if userid is None:
              return (False, None)
  
          # Compare the input authentication secret with the stored one.
          userid_profile_key = pytwis_constants.USER_PROFILE_KEY_FORMAT.format(userid)
          stored_auth_secret = self._rc.hget(userid_profile_key, pytwis_constants.AUTH_KEY)
          if auth_secret == stored_auth_secret:
              return (True, userid)
  
          # TODO: Resolve the inconsistency of the two authentication secrets.
          return (False, None)