public static function scope($name, $conditions)
    {
        $cls = get_called_class();

        if (!array_key_exists($cls, self::$_scopes)) {
            self::$_scopes[$cls] = array();
        }

        if (!array_key_exists($name, self::$_scopes[$cls])) {
            self::$_scopes[$cls][$name] = array();
        }

        self::$_scopes[$cls][$name] = $conditions;
    }