public function addDeferJs($asset, $priority = 10, $pipeline = true, $group = 'head')
    {
        user_error(__CLASS__ . '::' . __FUNCTION__ . '() is deprecated since Grav 1.6, use dynamic method with [\'loading\' => \'defer\']', E_USER_DEPRECATED);

        return $this->addJs($asset, $priority, $pipeline, 'defer', $group);
    }