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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.
It's because anonymous functions like that are very much inspired by the lambda calculus where you'd write λx. f x for python's lambda x: f(x) and it's just a "pythonization" of that syntax.