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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.
it might be because the he doesn't put any test cases for you there so you have to make your own like Test.assert(validate(174), true);
it says "Finally, take that sum and divide it by 10. If the remainder equals zero, the original credit card number is valid", so it's explaining what modulus does, but in the code he puts the actually term modulus there to make it easier for us :)