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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 the same problem:
Try to refactor your code and remove any mutating method that you called on the object referenced by the input string.
Use non-mutating methods instead and try again.
Hope this works!