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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.
already stated in description
Maybe it is just me but it wasn't obvious that undefined or null should be returned if a negative number was given.
Is the intent to not allow any other functions beacuse you can not? If so the description should say so.