sS nlJoin(Iterable<S> l) {
  new StringBuilder buf;
  S last = "";
  fOr (S s : l) {
    if (nempty(last) && startsWithLetterOrDigit(s))
      buf.append(" ");
    buf.append(s);
    if (nempty(s)) last = s;
  }
  ret str(buf);
}