static class AIEngine {
  new L<S> sentences;
  new Set<S> sentenceSet;
  new Map<S, O> objects;
  new IdentityHashMap<O, S> objectToID;
  long objectIDCounter;
  
  S addObject(O o) {
    S id = objectToID.get(o);
    if (id == null) {
      id = "o" + ++objectIDCounter;
      objects.put(id, o);
      objectToID.put(o, id);
    }
    return id;
  }
  
  S remember(S sentence) {
    if (!sentenceSet.contains(sentence)) {
      sentenceSet.add(sentence);
      sentences.add(sentence);
      return addObject(sentence);
    }
    return null;
  }
  
  void show() {
    for (S s : sentences)
      print("  " + s);
  }
  
  <A> A findObjectOfType(Class<A> c) {
    for (O o : objects.values())
      if (o.getClass() == c)
        return (A) o;
    return null;
  }
}