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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.
This version does not pre-suppose that b>a, nor that either integer lies in range(0,len(items)).
Total lack of a for loop should make this code run fast, nice work.