[Someone]

"think mod-13 arithmetic and prime numbered cards behaving differently"

I know it puts me firmly in the Mathematicians/CS types corner, but when I read that, I did not even have to think consciously to have 'once you are in Z13, prime numbers should not bother you anymore' spring up in my mind.

For those not Mathematicians/CS types: modulo 13 arithmetic does not have primes. For example, 2 is not prime because 3 times 5 equals 2 mod 13, 3 is not prime because it equals 4 times 4 (and, by extension, -4 times -4, which is 9 times 9), etc.

[anthonyb]

Hmm, well that's not how we played it - the cards incremented, not numbers themselves.

The rules went something like: when you play 'X', then all cards are incremented by 1. Kings are 13 and wrap around to Ace, which is 1.

The second rule would have been, say, all primes are spades, so King + 2 would be 2, which is prime. King of clubs on a three of hearts... perfectly legit.