Group.invalidNamingAttr("extension", extn);
  protected static void invalidStemOrExtn(String stem, String extn) {    if        (stem == null) {      throw new RuntimeException("null stem");    } else if (stem.equals("")) {      throw new RuntimeException("blank stem");    }    if        (extn == null) {      throw new RuntimeException("null extension");    } else if (extn.equals("")) {      throw new RuntimeException("blank extension");    }  }