public static function exch($cx, $ch, $vars, &$options) {
        $args = $vars[0];
        $args[] = $options;
        $e = null;
        $r = true;

        try {
            $r = call_user_func_array($cx['helpers'][$ch], $args);
        } catch (\Exception $E) {
            $e = "Runtime: call custom helper '$ch' error: " . $E->getMessage();
        }

        if($e !== null) {
            static::err($cx, $e);
        }

        return $r;
    }