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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.
At the beginning I was very confused why this counts as a solution, as it throws an error once submitting a string with only valid letters.
However the kata description at the end says "The string has a length greater or equal to one..." so I guess thats why this solution counts :P