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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.
Yup. This is an old kata, the difficulty is rather exaggerated by today's standards. Try and find another 3kyu kata with 9k solves.
Wait, does that mean all the tests are sudokus that can be solved by filling in naked singles? I know the description says that the tests are "easy" but I didn't think that was what was meant. Implemented a general sudoku brute-forcer instead (seems fast enough on random "hard" sudokus from the web). I guess it would be better to correct the description, but the author was online in 2017 for the last time, so I'm not sure that is possible.