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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.
Could you provide an example at least? Did you read that once reversed they should be a different number? So 11 is not a valid result, neither 2, 3, 5 nor 7. If you're sure there is the problem with the tests, provide your code and the language you're working on, the problem may exist only in that one.