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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.
But that would be much more inefficient: pre-computing the required array is mere memoization and that works better; same goes for defining auxiliary functions outside, so that every time the function run do not need to be recreated.
It is an extremely efficient solution, plus forcing myself to one-liner is great practice :)