protected function _prime()
    {
        if ($this->value < 2) {
            return false;
        }

        if (!$this->_decimal()->zero) {
            throw new \RuntimeException(
                'You cannot test if number is prime numer if it is not an integer'
            );
        }

        $max = floor(sqrt($this->value));

        for ($i = 2; $i <= $max; $i++) {
            if ($this->value % $i == 0) {
                return false;
            }
        }

        return true;
    }