// returns a prefiltered list of elements; you still need to do a
// full-text search on those.
// If it returns null, you have to search all elements
static <A> Cl<A> doubleWordIndex_lookupString(DoubleWordIndex<A> index, S query, O... _) {
  optPar bool debug;
  L<IntRange> ranges = index.wordRanges(query);
  if (empty(ranges)) null;
  new L<Set<A>> sets;
  
  for (IntRange r : ranges) { // go through words in query
    S word = substring(query, r);
    Set<A> l;
    if (r.start == 0)
      if (r.end == l(query)) // bad case - look for part of word
        l = msmGetAllAsSet(index.index1.index, containingIC(index.index1.words(), word));
      else { // look for ending of word - use reverse index
        Cl<S> words = prefixSubSet(index.index2.words(), reversed(word));
        l = msmGetAllAsSet(index.index2.index, words);
        if (debug)
          print(n2(words) + "/" + nEntries(l) + " for: " + word);
      }
    else if (r.end == l(query)) { // look for start of word
      Cl<S> words = prefixSubSet(index.index1.words(), word);
      l = msmGetAllAsSet(index.index1.index, words);
      if (debug)
        print(n2(words) + "/" + nEntries(l) + " for: " + word);
    } else // look for complete word
      l = index.get(word);
    if (empty(l)) ret l;
    sets.add(l);
  }
  Set<A> smallest = smallestCollection(sets), out = smallest;
  for (Set<A> set : sets) {
    if (set == smallest) continue;
    new Set<A> out2;
    for (A a : out)
      if (set.contains(a))
        out2.add(a);
    out = out2;
  }
  ret out;
}