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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 had a similar moment @crosssh, the test cases are a little ambiguous, if you comment out the first test case and remove your join you will see what is going on.
There is no issue with the kata, you get that error with the second test, not the first one. Check it on repl.it