BddTree getBlock(BddTree t, int low, int high) {
protected BddTree getBlock(BddTree t, int low, int high) {
    BddTree getBlock(BddTree t, int low, int high) {        if (t == null) return null;        for (BddTree p = t; p != null; p = p.next) {            if (p.first == low && p.last == high) return p;            BddTree q = getBlock(t.nextlevel, low, high);            if (q != null) return q;        }        return null;    }