docs: def Authenticate(self, app_id, challenge_data,
                     print_callback=sys.stderr.write):
      """See base class."""
      for authenticator in self.authenticators:
        if authenticator.IsAvailable():
          result = authenticator.Authenticate(app_id,
                                              challenge_data,
                                              print_callback)
          return result
  
      raise ValueError('No valid authenticators found')