static Collection<S> ai_gc_unreachedWords() {
  Set<S> unreached = caseInsensitiveSet();
  
  for (S term : indexedTerms())
    for (WebNode node : indexedNodes(term)) {
      S text = web_text(node);
      continue if unreached.contains(text);
      S d = ai_text(text, "has GC distance", "$X");
      if (d == null)
        unreached.add(text);
    }
    
  ret unreached;
}