static Map<S, LS> lcMergeStringsOccurringOnce(Map<S, LS> encodings) {
  new MultiSet<S> ms;
  for (LS l : values(encodings))
    ms.addAll(l);
  ret mapValues(encodings, l -> {
    l = cloneList(l);
    int i = 0;
    while (i < l(l)-1) {
      int j = i;
      while (j < l(l) && ms.get(l.get(j)) == 1) ++j;
      if (j > i+1)
        replaceSublist(l, i, j, ll(joinSubList(l, i, j)));
      i++;
    }
    ret l;
  });
}