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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 already did above when I closed the issue. ;)
note: if not already done, you should read about/understand what you did to spot the pattern if you encounter it again (or better, to avoid it) => read a bit about mutation of data structures (arrays, objects, ...)
I bet you are mutating the input. I changed something in the tests, that should be good now (reload the trainer page if needed)
edit: you really shouldn't get that very bad habit of mutating the input... for the very reason you just faced
in what part?