docs: public static String format(String[] d, String sep) {
      if(d == null) {
        return "null";
      }
      if(d.length == 0) {
        return "";
      }
      if(d.length == 1) {
        return d[0];
      }
      int len = sep.length() * (d.length - 1);
      for(String s : d) {
        len += s.length();
      }
      StringBuilder buffer = new StringBuilder(len)//
          .append(d[0]);
      for(int i = 1; i < d.length; i++) {
        buffer.append(sep).append(d[i]);
      }
      return buffer.toString();
    }