Ruby Challenges - Solving Every Sudoku

This topic is for the [Solving Every Sudoku][] exercise in the [Ruby Challenges][] trail. Post any questions, corrections, or pointers you have to share with other Upcase subscribers.
[Solving Every Sudoku]: https://exercises.upcase.com/exercises/solving-every-sudoku
[Ruby Challenges]: https://thoughtbot.com/upcase/ruby-challenges