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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've got the same problem. Also at least 3 kata has ended with such error. Maybe someone could explain what is the problem?
There should be an information in the description of this kata that the returned collection must be ordered by "a" from each pair [a,b]. It's useful when the solution has more than 2 elements.