// dictionary has to be a ciSet
static Chain<S> constructWordFromCISet(S word, Set<S> dictionary) {
  if (contains(dictionary, word)) ret Chain<S>(word);
  for (int i = 1; i < l(word); i++) {
    S prefix = takeFirst(i, word);
    if (contains(dictionary, prefix)) {
      Chain<S> chain = constructWordFromCISet(substring(word, i), dictionary);
      if (chain != null)
        ret Chain<S>(prefix, chain);
    }
  null;
}