// 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> wordIndex_lookupString(WordIndex<A> index, S query) {
  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))
        l = msmGetAllAsSet(index.index, containingIC(index.words(), word));
      else
        l = msmGetAllAsSet(index.index, endingWithIC(index.words(), word));
    else if (r.end == l(query))
      l = msmGetAllAsSet(index.index, prefixSubSet(index.words(), word));
    else
      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;
}