关于 make_excerpt($topic->body) 这个方法在哪里？

namespace App\Observers;

use App\Models\Topic;

// creating, created, updating, updated, saving,
// saved,  deleting, deleted, restoring, restored

class TopicObserver
{
    public function saving(Topic $topic)
    {
        $topic->body = clean($topic->body, 'user_topic_body');

        $topic->excerpt = make_excerpt($topic->body);
        这个方法在哪里？
    }
}