protected function addIncludesDefault($relation, $value = null)
    {
        $includes = array_get($this->info->includes, 'default', []);

        if (null !== $value) {
            $includes[ $relation ] = $value;
        } elseif ( ! array_key_exists($relation, $includes) && ! in_array($relation, $includes)) {
            $includes[] = $relation;
        }

        $this->info->includes['default'] = $includes;

        return $this;
    }