static String join_flex(L<S> glues, Iterable<String> strings) {
  if (strings == null) ret "";
  new StringBuilder buf;
  Iterator<S> i = strings.iterator();
  Iterator<S> iGlues = glues.iterator();
  if (i.hasNext()) {
    buf.append(i.next());
    while (i.hasNext())
      buf.append(iGlues.next()).append(i.next());
  }
  return buf.toString();
}