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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.
The real test cases (the ones that get executed when you press ATTEMPT) are much bigger than the example test cases (the ones you have passed). As you see in the error message, you had a stack overflow. If you dont know what that is, this might help: https://stackoverflow.com/questions/214741/what-is-a-stackoverflowerror. Also, I dont think this is an issue, it is a question.