static WebNode ai_tripleRefToWebNode(TripleRef<S> r) {
  if (r == null) null;
  Web web = webFromTriple(r.triple());
  try {
    ret web.nodes.get(r.position());
  } catch e {
    print("ai_tripleRefToWebNode: " + e + " " + sfu(r.triple()) + " => " + webToStringShort(web));
    throw rethrow(e);
  }
}