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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.
digits is a string, not a Fixnum
I agree with @awolfson. Check out Confident Ruby (http://devblog.avdi.org/2013/08/26/confident-ruby-is-finished/) for a more detailed look on how to structure code in a way to avoid checking arguments on multiple levels
No. The name does help provide a clue about how to think about the problem, but you need not know anything of physics.