static WebTree web_makeTree(WebNode node) {
  if (node == null) null;
  WebTree tree = new(node);
  web_makeTree_impl(tree, new HashSet);
  ret tree;
}

static void web_makeTree_impl(WebTree tree, HashSet<WebNode> seen) {
  if (!seen.add(tree.node)) ret;
  for (WebNode n : web_forwardRelated(tree.node)) {
    WebTree tree2 = new(n);
    tree.children.add(pair(tree.node.web.getRelation(tree.node, n), tree2));
    web_makeTree_impl(tree2, seen);
  }
}