def authenticated_userid(request):
    """ Return the authenticated :term:`userid` or ``None`` if
        no authenticated userid can be found. This method of the
        policy should ensure that a record exists in whatever
        persistent store is used related to the user (the user
        should not have been deleted); if a record associated with
        the current id does not exist in a persistent store, it
        should return ``None``.

        """
