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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.
I would like some clarification also. I was getting correct results for {a: null} also, but then this [3,6,9,null] thing. A test for that specific case is needed to show what you mean by that.
I agree. If I pass a list into someone else's function, I want at least a warning in the docs if it's going to modify it.