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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.
My initial solution passed even though I only checked arr1 to see if it was empty. I suggest adding a fixed test that provides empty nested arrays for arr2 as well.