// 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;
  int nRanges = l(ranges);
  new Map<A, MultiSetMap<Int>> theMap; // for every snippet, a map of string position to word index

  // special case, just a single word in query
  if (l(ranges) == 1 && first(ranges).start == 0 && first(ranges).end == l(query)) {
    new L<A> out;
    for (S fullWord : containingIC(index.index1.words(), query))
      for (WithIntArray<A> entry : index.index1.get(fullWord))
        out.add(entry!);
    ret out;
  }
  
  for iWord over ranges: { // go through words in query
    IntRange r = ranges.get(iWord);
    S word = substring(query, r);
    Cl<S> l; // all matching words in index
    if (r.start == 0) { // look for ending of word - use reverse index
      l = prefixSubSet(index.index2.words(), reversed(word));
      if (empty(l)) ret emptyList();
      
      // special loop that accounts for length of actual word
      new MultiSetMap<A, Int> positions;
      for (S fullWord : l)
        for (WithIntArray<A> entry : index.index1.index.get(fullWord))
          positions.putAll(entry!, map(i -> i+l(fullWord)-l(word), toList(entry.array)));
          
      for (WithIntArray<A> entry : index.index1.index.get(fullWord)) {
        MultiSetMap<Int> msm = theMap.get(entry!);
        if (msm == null) theMap.put(entry!, msm = new MultiSetMap);
        int ofs = l(fullWord)-l(word)-r.start;
        for (int i : entry.array)
          if (i+ofs >= 0)
            msm.put(i+ofs, iWord);
      }
      continue;
    } 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)) {
        MultiSetMap<Int> msm = theMap.get(entry!);
        if (msm == null) theMap.put(entry!, msm = new MultiSetMap);
        for (int i : entry.array)
          if (i-r.start >= 0)
            msm.put(i-r.start, iWord);
      }
  }
  
  for i over ranges: {
    IntRange range = ranges.get(i);
    MultiSetMap<A, Int> positions = multiSetMaps.get(i);
  }
  
  new L<A> out;
  theLoop: for (A snippet, MultiSetMap<Int> msm : theMap) {
    for (int position, L<Int> wordIndices : msm.data)
      if (l(wordIndices) == nRanges) {
        out.add(snippet);
        continue theLoop;
      }
  }
  ret out;
}