Sometimes there are some configuration values which have both to be set in combination. So you want to check if both or none of them are set. Usually this would be done using XOR. But unfortunately there isn't such an operator in Puppet Language at the moment.
Of course there are alternative ways to get to the same result:
(($a and !$b) or ($b and !$a))
But a native XOR operator would make it much easier and better to understand:
$a xor $b
So I think it would be very much appreciated if this operator will be implemented in Puppet Language.