public function tag($abstracts, $tags)
    {
        $tags = is_array($tags) ? $tags : array_slice(func_get_args(), 1);

        foreach ($tags as $tag) {
            if (! isset($this->tags[$tag])) {
                $this->tags[$tag] = [];
            }

            foreach ((array) $abstracts as $abstract) {
                $this->tags[$tag][] = $abstract;
            }
        }
    }