t.firstLevel = bddvar2level[low]; t.lastLevel = bddvar2level[high];
    void update_seq(BddTree t) {        int n;        int low = t.first;        for (n = t.first; n <= t.last; n++)            if (bddvar2level[n] < bddvar2level[low])                low = n;        for (n = t.first; n <= t.last; n++)            t.seq[bddvar2level[n] - bddvar2level[low]] = n;    }