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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.
As others have pointed out. There is no guidence on what should be returned if the input is invalid. -1 maybe? Would have made the Kata more interesting.
If there is no guidance or it doesn't test for it, why even mention it.