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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.
No idea if it was added after your post, but:
Think of it as any method that returns either a 1 or a 0. The implementation of it in this case is arbitrary since it is implemented by the caller and not within the method that is being tested.
I think you should state something along the fact that the interval is open, not closed. That through me off because my implementation was including the endpoints of the interval, not realizing that the interval was open.