// assumes indices are sorted
static L<S> splitStringAtIndices(S s, L<Int> indices) {
  Iterator<Int> it = indices.iterator();
  new L<S> parts;
  int i = 0;
  while true {
    if (!it.hasNext()) {
      parts.add(substring(s, i));
      break;
    }
    int j = it.next();
    parts.add(substring(s, i, j));
    i = j;
  }
  ret parts;
}