static LS stringToChunksWithOverlap(S s, int chunkLength, int overlap) {
  assertTrue(overlap < chunkLength);
  assertTrue(chunkLength > 0);
  int n = l(s), i = 0;
  new LS out;
  while (i < n) {
    out.add(substring(s, i, i+chunkLength));
    i += chunkLength-overlap;
  }
  ret out;
}