static CharInToken charIndexToCharInToken(LS tok, int charIndex) {
  int i = 0, idx = 0, lTok;
  while (i < l(tok) && idx+(lTok = l(tok.get(i))) <= charIndex) {
    idx += lTok;
    i++;
  }
  ret CharInToken(tok, i, charIndex-idx);
}