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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
ATM PIN codes have not meaningfully changed format since 1967. YAGNI.
Unless you want to maintain some dead code or another too-generic hell,
you probably don't want to anticipate that much, especially on such a simple piece of code.
That is what uses cases and product rules are for:
if a rule is meant to be changed at a later point, you simply rewrite the rule and delete the old one.
Also you can't be sure that a rule is meant to evolve - it can be discussed at some point and then it can be cancelled - or the rule is not meant to be changed at all.
The same logic applies to this kata: the description gives a simple rule, kata are not meant to be changed after leaving beta, so the rule is not likely changing at any point.
its a 7 kyu challenge, wdym?