public InvariantStatus add_modified(long[] a1, long[] a2, int count) {
   InvariantStatus is = check_modified(a1, a2, count);
   if ((is == InvariantStatus.FALSIFIED) && Debug.logOn())
     log(format() + " destroyed by " + Debug.toString(a1) + " " + Debug.toString(a2));
   return (is);
 }