public function setShufflingGroups(ShufflingGroupCollection $shufflingGroups) 
    {
        if (count($shufflingGroups) === 0) {
            $msg = "A Shuffling object must be composed of at least 1 ShufflingGroup object. None given";
            throw new InvalidArgumentException($msg);
        } elseif (($c = count($shufflingGroups)) > 2) {
            $msg = "A Shuffling object must be composed of at most 2 ShufflingGroup objects. ${c} given.";
            throw new InvalidArgumentException($msg);
        } else {
            $this->shufflingGroups = $shufflingGroups;
        }
    }