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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 think it is what Chrono said.
I tried using the .remove method first and it passed the tests and failed the random. Then I changed to another method and managed to pass in everything.
Same here.
I even tried the code, with different values, on Repl.it and it gave me the right answer each time. Maybe a bug or something like it?