sclass TripleIndex extends VirtualNodeIndex {
  SyncListMultiMap<S, TripleRef<S>> index = caseInsensitiveSyncListMultiMap();
  Map<GlobalID, TripleWeb> websByID = synchroHashMap();
  bool activated; // are we the main index (see tripleIndex())

  int numWebs() {
    ret l(websByID);
  }
  
  L<WebNode> get(S query) {
    ret ai_tripleRefsToWebNodes_lazyList(index.get(query));
  }
  
  Web getWeb(GlobalID id) {
    ret webFromTriple(websByID.get(id));
  }
  
  void addWeb(Web web) {
    if (web == null) ret;
    TripleWeb w = ai_webToTripleWeb(web);
    if (w == null)
      //ret with print("Skipping non-tripelizable web");
      fail("Skipping non-tripelizable web: " + webToStringShort(web));
    index.put(w.a, TripleA(w));
    index.put(w.b, TripleB(w));
    index.put(w.c, TripleC(w));
    websByID.put(w.globalID, w);
  }
  
  void removeWeb(Web web) {
    if (web == null) ret;
    TripleWeb w = ai_webToTripleWeb(web);
    if (w == null) ret;
    websByID.remove(w.globalID);
    indexRemove(w.a, w.globalID);
    indexRemove(w.b, w.globalID);
    indexRemove(w.c, w.globalID);
  }
  
  // internal
  void indexRemove(S term, GlobalID globalID) {
    L<TripleRef<S>> l = index.get(term);
    for i over l:
      if (eq(((TripleWeb) l.get(i).triple).globalID, globalID)) {
        index.remove(term, l.get(i));
        ret;
      }
  }
  
  void clear() {
    index.clear();
    websByID.clear();
  }
  
  void activate {
    if (!activated) {
      activated = true;
      ai_onNewOrRemovedWeb(func(Web web) {
        addWeb(web);
        false;
      }, func(Web web) {
        removeWeb(web);
        false;
      });
    }
  }
}