public static function makeNodeUrlsRelative(Node $node, $baseUrl, $recursive=true) {
    // use NodeValueIterator to iterate over all Node values
    // and call the global convert function on each
    $iter = new NodeValueIterator($node, $recursive);
    for($iter->rewind(); $iter->valid(); $iter->next()) {
      self::makeValueUrlsRelative($iter->currentNode(), $iter->key(), $baseUrl);
    }
  }