private function getSibling($offset)
    {
        $siblingsAndSelf = $this->parent->getChildren();
        $pos             = array_search($this, $siblingsAndSelf);
        if (isset($siblingsAndSelf[$pos + $offset])) {
            return $siblingsAndSelf[$pos + $offset]; // Next / prev. node
        }
        return null;
    }