return this;
        public void restrictWith(BDD that) {            int x = _index;            int y = ((bdd) that)._index;            int a = bdd_restrict(x, y);            //System.out.println("restrictWith("+System.identityHashCode(this)+") "+x+" -> "+a);            bdd_delref(x);            if (this != that)                that.free();            bdd_addref(a);            this._index = a;        }