static Collection<Symbol> ai_expandShortenedKeys_multi(Collection<Symbol> l) {
  new LinkedHashSet<Symbol> set;
  if (l != null) for (Symbol sKey : l) if (sKey != null) {
    if (!ai_possiblyShortenedKey(sKey))
      set.add(sKey);
    else
      for (int position = 0; position < 3; position++)
        for (TripleRef<Symbol> r : unnull(tripleIndex().getTripleRefs(position, sKey)))
          if (r != null) set.add(r.get());
  }
  ret set;
}