public function slug($slug = null): self
    {
        if (! empty($slug)) {
            // set this slug to be used in with callback
            $this->slug = $slug;

            // exception to filter on specific slug
            $exception = function ($query) {
                $query->where('slug', '=', $this->slug);
            };

            // load term to filter
            return $this->whereHas('term', $exception);
        }

        return $this;
    }