static void web_printTree(WebNode node) {
  web_printTree(node, "", "", new HashSet);
  print();
}

static void web_printTree(WebNode node, S indent, S relation, HashSet<WebNode> seen) {
  if (node == null) ret;
  print(indent + relation + node.text());
  if (!seen.add(node)) ret;
  for (WebNode n : web_forwardRelated(node))
    web_printTree(n, indent + "  ", appendIfNempty(web_relationText(node, n), " -> "), seen);
}