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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.
It doesn't matter if it's trivial, the users can't understand which input is valid and which is not. And no, you can't derive from the Kata's instructions that the string should ONLY contain those characters.
Same here, tests fail in C#