static Collection<WebNode> web_transitiveForwardRelated(WebNode node) {
  new HashSet<WebNode> seen;
  new HashSet<WebNode> nu;
  nu.add(node);
  while (nempty(nu)) {
    WebNode n = first(nu);
    nu.remove(n);
    seen.add(n);
    for (WebNode x : web_forwardRelated(n))
      if (!seen.contains(x))
        nu.add(x);
  }
  ret seen;
}