@Contract(pure = true)
  private static String concatWithEx(List<String> s, String concator, int N, String NthConcator) {
    StringBuilder result = new StringBuilder();
    int i = 1;
    for (Iterator iterator = s.iterator(); iterator.hasNext(); i++) {
      String str = (String)iterator.next();
      result.append(str);
      if (iterator.hasNext()) {
        if (i % N == 0) {
          result.append(NthConcator);
        }
        else {
          result.append(concator);
        }
      }
    }
    return result.toString();
  }
@Contract(pure = true)
  private static String concatWithEx(List<String> s, String concator, int N, String NthConcator)