def unauthorized(id, request):
    """Signal an authorization failure.

        This method is called when an auhorization problem
        occurs. It can perform a variety of actions, such
        as issuing an HTTP authentication challenge or
        displaying a login interface.

        Note that the authentication utility nearest to the
        requested resource is called. It is up to
        authentication utility implementations to
        collaborate with utilities higher in the object
        hierarchy.

        If no principal has been identified, id will be
        None.
        """
