docs: def check_application_state(self, request, callback):
          "Check optional state parameter."
          stored = request.session.get(self.session_key, None)
          returned = request.GET.get('state', None)
          check = False
          if stored is not None:
              if returned is not None:
                  check = constant_time_compare(stored, returned)
              else:
                  logger.error('No state parameter returned by the provider.')
          else:
              logger.error('No state stored in the sesssion.')
          return check