static MultiSet<S> ai_allPrefixesWithWordCount(S s) {
  MultiSet<S> words = asCIMultiSet(words2_noNumbers(s));
  MultiSet<S> out = ciMultiSet();
  for (S prefix : ai_allWordPrefixes(s)) {
    Set<S> matching = prefixSubSet((NavigableSet) words.map, prefix);
    out.add(prefix, multiSetTotalCountForKeys(words, matching));
  }
  ret out;
}