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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.
+1 to this comment - a whole bunch of the top-rated answers actually fail on this check, so it would be a great idea to add this in to the test cases.
Technically correct; however, the original list was mutated. Try deep-copying the list that was passed in, then work with those values instead. :)