// 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> deepDoubleWordIndex_lookupString(DoubleWordIndex<WithIntArray<A>> index, S query, O... _) {
  optPar bool debug;
  L<IntRange> ranges = index.wordRanges(query);
  if (empty(ranges)) null;
  new L<MultiSetMap<A, Int>> multiSetMaps; // snippet & positions for each word in query
  
  for (IntRange r : ranges) { // go through words in query
    S word = substring(query, r);
    Cl<S> l; // all matching words in index
    if (r.start == 0)
      if (r.end == l(query)) { // bad case - look for part of word
        l = containingIC(index.index1.words(), word);
      } else { // look for ending of word - use reverse index
        l = prefixSubSet(index.index2.words(), reversed(word));
      }
    else if (r.end == l(query)) { // look for start of word
      l = lmap reversed(prefixSubSet(index.index1.words(), word));
    } else // look for complete word
      l = ll(word);
      
    if (empty(l)) ret emptyList();
    
    // merge all positions  
    new MultiSetMap<A, Int> positions;
    for (S fullWord : l)
      for (WithIntArray<A> entry : index.index1.index.get(fullWord)
        positions.putAll(entry!, toList(entry.array));
      
    multiSetMaps.add(positions);
  }
  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;
}