class BadConvert {
  public static Object up (LinkedList<String> xs) {
    return (LinkedList<Object>) xs; // compile-time error
  }
  public static LinkedList<String> down (Object o) {
    if (o instanceof LinkedList<String>) // compile-time error
      return (LinkedList<String>)o; // compile-time error
    else throw new ConvertException();
  }
}