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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.
What I'm reading in the description is the input string can only contain braces of three sorts, and nothing else.
The kata tests support this understanding.
So I'd say you're trying to see this kata as something more than it actually is. :)