private function getBlankNodeId($id = null)
    {
        if ((null !== $id) && isset($this->blankNodeMap[$id])) {
            return $this->blankNodeMap[$id];
        }

        $bnode = '_:b' . $this->blankNodeCounter++;
        $this->blankNodeMap[$id] = $bnode;

        return $bnode;
    }