public function group($group)
    {
        if ($group instanceof Closure) {
            $callback = $group;
            $group = new Group();
            is_callable('Closure::bind') && $callback = $callback->bindTo($group, $group);
            $callback($group);
        }

        if ($group instanceof Group) {
            $group = $group->getGroup();
        }

        $this->pipeline[] = ['$group' => $group];

        return $this;
    }