private static void writeRoots(Element element, Collection<? extends TreeItem<Pair<AbstractUrl, String>>> roots) {
    for (TreeItem<Pair<AbstractUrl, String>> root : roots) {
      final AbstractUrl url = root.getData().getFirst();
      if (url == null) continue;
      final Element list = new Element(FAVORITES_ROOT);
      url.write(list);
      list.setAttribute(CLASS_NAME, root.getData().getSecond());
      element.addContent(list);
      final List<TreeItem<Pair<AbstractUrl, String>>> children = root.getChildren();
      if (children != null && !children.isEmpty()) {
        writeRoots(list, children);
      }
    }
  }
private static void writeRoots(Element element, Collection<? extends TreeItem<Pair<AbstractUrl, String>>> roots)