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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.
Before using any Ruby features, you should check since which version of Ruby they're available. And definitely not asserting any "errors" on arbitrary "not for production" excuses, it's not helpful at all ;-)
In this case, it's only available since 2.3.