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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.
Unfortunately, this is not tail-recursive. All of the intermediate calls need to be kept on the stack because they still have work to do, specifically: adding 2 to the result of the recursive call they are waiting on.