static void web_printWebTree(WebTree tree) {
  web_printWebTree(tree, "", "");
  print();
}

static void web_printWebTree(WebTree tree, S indent, S relation) {
  if (tree == null) ret;
  print(indent + relation + tree.node.text());
  for (Pair<WebNode, WebTree> p : tree.children)
    web_printWebTree(p.b, indent + "  ", appendIfNempty(p.a.text(), " -> "));
}