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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 can always just print the values you get.
I think you shouldn't require an empty string if the book list is empty - you have 0 books of each category, so the solution should be "(A : 0) - (B : 0) - (G : 0)", or whatever the testcase was.
I think you are understood the challenge wrong. You are only supposed to look at the first letter.