docs: public static <X> String join(Iterable<X> l, String glue) {
      StringBuilder sb = new StringBuilder();
      boolean first = true;
      for (X o : l) {
        if ( ! first) {
          sb.append(glue);
        } else {
          first = false;
        }
        sb.append(o);
      }
      return sb.toString();
    }