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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.
basically as written, built in function array_sum will be used on call of sum function. You can find documentation of array_sum and you'll see that it waits for array parameter