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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.
Which language? For Python, the idea is that you should return a generator expression - not sure what they're called in Haskell and JavaScript but they do have similar ideas.
See link in reply to comment below for more details.