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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.
The instructions state you won't get negative numbers on input, therefore IMO it's like a contract and you really don't need to count with negative numbers in this case. This is why the kata tests also pass because they won't send you any single negative number. Also check the similar answer from @Chrono79 below.
The exercise instructions clearly state this, which makes it a requirement.
This is correct since the acceptance criteria don't expect negative numbers as input parameter. We are not trying to do classic sad path validation.
Not exists validation for negative numbers