public function links($relType, $urn = null)
    {
        $rawLinks = [];
        if (!empty($this->links->$relType)) {
            $rawLinks = $this->links->$relType;
        }
        $links = new CollectionDocJsonLinks($rawLinks, $this->_auth);

        // optionally filter by urn
        if ($urn) {
            return $links->rels([$urn]);
        } else {
            return $links;
        }
    }