final protected function init(string $a, string $b, & $aDig, & $bDig, & $aNeg, & $bNeg) : void
    {
        $aNeg = ($a[0] === '-');
        $bNeg = ($b[0] === '-');

        $aDig = $aNeg ? \substr($a, 1) : $a;
        $bDig = $bNeg ? \substr($b, 1) : $b;
    }