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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.
Overall, a fun challenge, though I agree with those who are saying the test for the empty list goes against the instructions, as it is not n x n. Yes, there is a note about it and the fix is pretty trivial, but trying to explain it away as a design choice is just silly. It's a bad design choice.