repoze.bfg confusion

gistfile1.txt

        if 'bfg.routes.matchdict' in environ:
            matchdict = environ['bfg.routes.matchdict']
            path = matchdict.get('traverse', '/')
            subpath = matchdict.get('subpath', '')
            subpath = tuple(filter(None, subpath.split('/')))
        else:
            # this request did not match a Routes route
            subpath = ()
            try:
                path = environ['PATH_INFO'] or '/'
            except KeyError:
                path = '/'

        try:
            .
            .
            .