static LS greedySplitIntoWordsCI(S s, Set<S> words) {
  words = asCISet(words);
  int i = 0, last = 0;
  new LS out;
  while (i < l(s)) {
    S word = longestPrefixInCISet(substring(s, i), words);
    if (nempty(word)) {
      addIfNempty(out, substring(s, last, i));
      out.add(word);
      last = i = i+l(word);
    } else ++i;
  }
  addIfNempty(out, substring(s, last, i));
  ret out;
}