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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.
The penny is beginning to drop. There is an inferred 'return' after an arrow, so your arrow functions return an arrow function. To begin with I was trying to read it as a function of two variables, which of course is different.