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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.
I completely agree. Every time I thought I had solved the exercise it turned out there was another edge case I hadn't accounted for. On top of that the first initial test cases are all arrays of arrays so I thought I had solved this relatively quickly. It's frustrating when the difficulty in a problem is not the problem itself rather stumbling through the poor instructions.