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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.
I think that in your case the comments don't make the code easier to read and understand otherwise your solution is clean. Maybe just less efficient since you used a dict instead of a set.
Also you did the same "mistake" i did, you can get rid of the else statment because you always want to add the checked value.