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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 with you, but I guess it's clearer than a tail-recursive solution, because I can only think of an additional function with an accumulator parameter for the list of digits to make it tail-recursive :)
Just a question as a noob functional programmer: wouldn't this be very inefficient even on a funtional language since it is not tail-recursive?