Hmmm, there seem to be indeed two different schemes here, both being referred to as "mod 10" in one way or the other. I think for now it would be good to finish up what we have, i.e. the Luhn one. With a follow-up ticket we could then create another constraint for the other scheme used by ISBN-13 etc. As you say I think it makes sense to have different constraints for this to keep things comprehensible.