/** Clear the counterexample. */
 public void clear() {
   for (JDDNode dd : states) {
     JDD.Deref(dd);
   }
 }
 /** Add a state to the path (as a BDD, which will be stored and Ref'ed). */
 public void addState(JDDNode state) {
   JDD.Ref(state);
   states.add(state);
 }