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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.
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How does this approach handle when the resulting "digit" after multiplying by 2 is greater than 9?
I also don't think this solution solves the Kata as written. This changes the class in order to make push a one line function. Since the original class declaration didn't have an argument for next I feel this misses the point.