lib/BN.php
public function isub(BN $num) {
        $this->bi = $this->bi->sub($num->bi);
        return $this;
    }
public function isub(BN $num) {
        $this->bi = $this->bi->sub($num->bi);
        return $this;
    }
public function isub ( BN $ num ) { $ this -> bi = $ this -> bi -> sub ( $ num -> bi ) ; return $ this ; }
Subtract `num` from `this` in-place
Subtract num from this in - place