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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.
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I was going to write the same thing, but I thought I should reassign a first(to the same list comprehension you use), because the question instructs "It should remove all values from list a, which are present in list b." so I ressaigned a and then returned it