abstract sclass VirtualNodeIndex {
  // concepts that are used in the generated webs,
  // but can't be queried
  new TreeSet<S> implicitConcepts;
  
  abstract int numWebs();
  abstract int numTerms();
  abstract L<WebNode> get(CharSequence query);
  bool hasShortTerm(S s) { false; }
  Web getWeb(GlobalID id) { ret null; }
  Collection<Symbol> indexedTerms() { null; }
  
  Collection<TripleWeb> allTriples() { null; }
  
  Set<S> implicitConcepts() { ret implicitConcepts; }
}