@Override
  public int compare(String s1, String s2)
  {
    if (s1.equals(s2))
      return 0;
    if ((s1.compareTo("NIL") == 0) || (s2.compareTo("LIN") == 0))
      return 1;
    if ((s2.compareTo("NIL") == 0) || (s1.compareTo("LIN") == 0))
     return -1;
    if (s1.compareTo(s2) < 0)
      return -1;
    else return 1;
  }//compare(String, String)