void validate(int k) { validate(k, -1);
void validate(int k, int lastLevel) { if (k < 2) return; int lev = LEVEL(k);  if (lev <= lastLevel) throw new BDDException(lev+" <= "+lastLevel);  validate(LOW(k), lev);  validate(HIGH(k), lev);
    void validate(int k) {        validate(k, -1);    }