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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.
Like the use of partition, but you are comparing the whole word instead of just the first letter like the specification states.
Nice one-liner, but you are wasting a lot of cycles repeatedly calling "word.chars.sort". I would recommend assigning that to a variable right off and then use that variable in your select.
What happens if "number" is 0, 1, or 2?
Very clean, I like the recursion, but you can't rely on a hash to maintain its ordering. I would recommend getting the keys, sorting them, then using each, or have a second with the roman numerals to use each on, since you can rely on the ordering of an array.