abstract sclass VirtualNodeIndex {
  // concepts that are used in the generated webs,
  // but can't be queried
  new TreeSet<S> implicitConcepts;
  
  abstract int numWebs();
  abstract L<WebNode> get(S query);
  Web getWeb(GlobalID id) { ret null; }
  
  Set<S> implicitConcepts() { ret implicitConcepts; }
}