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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.
I agree you should say return null, not nil, just to make it a little clearer
A better explanation and some more varied test cases might help. I struggled to understand what was being asked for.
The descriptions were not that clear on that dashes (-) were allowed only mentioned underscores