static Collection<S> ai_gc_unreachedWords() {
  Set<S> unreached = caseInsensitiveSet();
  
  Map<S, Double> distances = ciMap();
  for (WebRelation r : indexedRelations("has GC distance")) pcall {
    distances.put(web_text(r.a), parseDouble(web_text(r.b)));
  }
  
  for (S text : allIndexedFullTerms()) {
    continue if unreached.contains(text);
    if (!distances.containsKey(text))
      unreached.add(text);
  }
    
  ret unreached;
}