static MultiSet<S> ai_allPrefixesWithWordCount(S s) {
  TreeSet<S> words = asCISet(words2_noNumbers(s);
  MultiSet<S> out = ciMultiSet();
  for (S prefix : ai_allWordPrefixes(s))
    out.add(prefix, l(prefixSubSet(words, prefix)));
  ret out;
}