static S joinStrings(S sep, S... strings) {
  ret joinStrings(sep, Arrays.asList(strings));
}

static S joinStrings(S sep, Collection<S> strings) {
  new StringBuilder buf;
  for (S s : unnull(strings)) if (nempty(s)) {
    if (nempty(buf)) buf.append(sep);
    buf.append(s);
  }
  ret str(buf);
}