code: func (a *Authenticator) usersAPI() UsersAPI {
  	for _, m := range a.Methods {
  		if api, ok := m.(UsersAPI); ok {
  			return api
  		}
  	}
  	return nil
  }
docs: // usersAPI returns implementation of UsersAPI by examining Methods.
  //
  // Returns nil if none of Methods implement UsersAPI.