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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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It's about the diagonals (complete ones and partials ones). A very good explanation was made by benjaminzwhite in his python solution of the kata, I can't do better. ;)
Sorry for the question, but I can't understand the 2a(a-1)(3b-a-1)/3 and the corrispective 2b(b-1)(3a-b-1)/3.
I want an explanation, please.