@Nullable
  private static String concat(String[] strings, int headLimit, int tailLimit) {
    if (strings.length <= headLimit + tailLimit) return null;
    StringBuilder buffer = new StringBuilder();
    String separator = "";
    for (int i = headLimit; i < strings.length - tailLimit; i++) {
      buffer.append(separator);
      buffer.append(strings[i]);
      separator = File.separator;
    }
    return buffer.toString();
  }
@Nullable
  private static String concat(String[] strings, int headLimit, int tailLimit)