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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.
There have been many debates about the use of the oxford comma, but as far as "correctness" goes, they're both gramatically correct. However, the problem speficies that you return the answer without the oxford comma, so if you include it, your function is wrong.