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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 description is not really clear, more internal tests would help.
It happened to me with one of my test cases at first, then I had to change my code to accept BigInteger values in n. So I agree with you.