static WebNode ai_tripleToWebNode(TripleWeb w, int position) {
  if (w == null) null;
  Web web = webFromTripleWeb(w);
  try {
    ret web.nodes.get(position);
  } catch e {
    print("ai_tripleToWebNode: " + e + " " + sfu(r.triple()) + " => " + webToStringShort(web));
    throw rethrow(e);
  }
}