public function rank()
    {
        $eps = pow(2.0, -52.0);
        $tol = max($this->m, $this->n) * $this->s[0] * $eps;
        $r = 0;
        for ($i = 0; $i < count($this->s); ++$i) {
            if ($this->s[$i] > $tol) {
                ++$r;
            }
        }
        return $r;
    }