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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.
After seeing this solution, i learned that, in contrast to BST in other languages, BST(Set) in javascript can hold its insertion order.
This solution is clean and stable.
unpowerful
I don't like this solution because you are mutating the function argument.
powerful