docs: def addSubscriber(self, email):
          """Add a subscriber to this workitem
  
          If the subscriber has already been added, no more actions will be
          performed.
  
          :param email: the subscriber's email
          """
  
          headers, raw_data = self._perform_subscribe()
          existed_flag, raw_data = self._add_subscriber(email, raw_data)
          if existed_flag:
              return
  
          self._update_subscribe(headers, raw_data)
          self.log.info("Successfully add a subscriber: %s for <Workitem %s>",
                        email, self)