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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.
Added spoiler tag to your comment because it was giving away some code still useful to come up with an efficient solution; added in the description also to be ready to handle big numbers: telling anything more would end up spoonfeeding users toward the solution, I fear.
Thanks for your feed :)