static S ocr_joinAsGrouped(L<S> l) {
  new StringBuilder buf;
  for (S s : l)
    if (l(s) == 1 || l(s) == 2 && s.charAt(0) == '\\')
      buf.append(s);
    else
      buf.append("[").append(s).append("]");
  ret str(buf);
}