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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.
In the ::is_valid method I use an Integer instance method called digits on the number argument in order to break the number into an array of its constituent digits, and I get this error message: <NoMethodError: undefined method `digits' for 1:Fixnum>. But the Integer class is the parent of Fixnum, so the method 'digits' should be defined for the Fixnum 1. What gives? These tests pass in irb. Oh, by the way, I'm coding in Ruby.