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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.
But it doesnt say use recursion in a programming sense. It says Digital Root is a recursive sum mathematically (its also iterative). You dont have to use recursion to work it out though...
Saying that I like your solution its nice and easy to read.