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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.
So what about ostringstream? It's basically the same. You can define it in Preloaded and clear, check, print, whatever in the tests.
I don't think so, especially considering that testing results are printed to cout. But why do you want cout specifically? You can create a different string or file stream.