private static function calculateMeanMicroAmount($min, $max)
    {
        if ($min === null || $max === null) {
            return null;
        }
        if ($min->getMicroAmount() === null
            || $max->getMicroAmount() === null) {
            return null;
        }

        return ($min->getMicroAmount() + $max->getMicroAmount()) / 2;
    }