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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.
In case you haven't worked this out yet, [null] means an array with a null object inside it, whereas null means a null object. S0 you just want to change your code so it returns null if any of the elements are null.