protected function get_permission_strings() {
        static $strpermissions;
        if (!$strpermissions) {
            $strpermissions = array(
                CAP_INHERIT => new lang_string('inherit', 'role'),
                CAP_ALLOW => new lang_string('allow', 'role'),
                CAP_PREVENT => new lang_string('prevent', 'role'),
                CAP_PROHIBIT => new lang_string('prohibit', 'role')
            );
        }
        return $strpermissions;
    }