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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.
In the task description:
"Implement a function, which takes a non-negative integer"
So non-negative is assumed already. In production code this obviously would want some additional robustness, but based on the description of the task the solution fits.
The tests pass, and those are the final say anyway.