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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.
Didn't catch this when I wrote it, but the
+=in the body of themap.with_indexis unnecessary --+is all that's required to replace the current value in the row with the updated sum.