The rules seem to be that it checks if it's an available multiple of 7 first and then if it's invalid. I initially thought the rules would work the way you have described, but if that was the case, would all multiples of 7 and 3 be off limits?
That's how it initially was. But I have now made it so that, as you say, it first checks to see if it's a multiple of 7. If it's a multiple of both 7 and 3 (regardless of whether or not you have played it before), then you will no longer be penalized.
I think I understand why you decided to do that, but it might be worth an explanation modification. I tested my understanding of the rules by trying 21, and was surprised. Not that it's a big deal!
