static WebNode ai_tripleToWebNode(TripleWeb w, Symbol searchTerm, int occurrence) {
  if (w == null) null;
  ifdef ai_tripleToWebNode_debug
    print("ai_tripleToWebNode: " + sfu(w) + ", searchTerm=" + quote(searchTerm) + ", occurrence=" + occurrence);
  endifdef
  Web web = webFromTripleWeb(w);
  try {
    for (int i = 0; i < l(web.nodes); i++) 
      if (eqic(web_sym(web.nodes.get(i)), searchTerm)) {
        if (occurrence-- == 0) ret web.nodes.get(i);
      }
    null;
  } catch e {
    print("ai_tripleToWebNode: " + e + " " + sfu(w) + " => " + webToStringShort(web));
    throw rethrow(e);
  }
}