// 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;

}