public void replaceWith(BDDPairing pair) {
public BDD replaceWith(BDDPairing pair) {
        public void replaceWith(BDDPairing pair) {            int x = _index;            int y = bdd_replace(x, ((bddPair) pair));            //System.out.println("replaceWith("+System.identityHashCode(this)+") "+x+" -> "+y);            bdd_delref(x);            bdd_addref(y);            _index = y;        }