public function siblings()
    {
        if ($this->siblings !== null) {
            return $this->siblings;
        }
        $master = $this->master();
        if ($master === null) {
            // Todo: return all top-level objects.
            $siblings = [];
        } else {
            // Todo: Remove "current" object from siblings
            $siblings = $master->children();
        }
        $this->siblings = $siblings;
        return $this->siblings;
    }