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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.
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Thanks! I hadn't thought of vacuous truths before. Good to know for what to do in the future.
https://en.wikipedia.org/wiki/Vacuous_truth
Why do your random test cases say that it should return True if the letter never appears in the first place? (I would have intuitively thought that in such a case, the function should return false.) The problem description doesn't explain what we should do when a letter never appears.