def forget(environ, identity):
    """ On egress (challenge required):

        args -> [ (header-name, header-value), ...] | None

        Return a list of headers suitable for allowing the requesting
        system to forget the identification information (e.g. a
        Set-Cookie header with an expires date in the past).  Return
        None if no headers need to be set.  These headers will be
        included in the response provided by the challenge app.
        """
