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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.
I think it boils down to the problem of evaluation of arithmetical expression, minus tokenization. We already have several kata which require evaluation of a string expression, of various difficulty (from BEDMAS to math functions to REPL).