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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.
Agreed-- my solution was more complex than was necessary because I was filtering out letters-- which, if numbers are allowed, should not have passed.
The wording of this question states "letters" instead of "characters". but the tests are really checking for characters. The description should be ammended