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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.
Seems to me when it hit dead ends, which end in 1, it pops twice since range(1-1, 0, -1) skips the for loop. next loop starts with the poped value-1.
Old kata syndrome ;-)