static void ai_gc_scan(S root) {
  ai_gc_clear();
  time "GC Scan" {
    ai_postTriple(root, "has GC distance", "0");
    HashMap<S, Int> distances = lithashmap(root, 0);
    LinkedList<S> queue = litLinkedList(root);
    S word;
    while ping ((word = queue.poll()) != null) {
      int d = distances.get(word)+1;
      for (WebNode node : indexedNodes(word))
        for (WebNode n : web_forwardOrBackwardRelated(node)) {
          S text = ai_shortenForIndex(web_text(n));
          Int od = distances.get(text);
          if (od == null || d < od) {
            distances.put(text, d);
            queue.add(text);
          }
        }
    }
  }
  print(l(distances) + "/" + l(indexedTerms()) + " terms were reached in GC scan from " + quote(root) + ".");
}