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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.
You should read the task again and understand the pattern of the examples. Currently your code is only removing all non-letters from the input (so for the input range of this kata it is actually the identity function).