Ruby: class_eval vs instance_eval

https://stackoverflow.com/questions/900419/how-to-understand-the-difference-between-class-eval-and-instance-eval 

https://stackoverflow.com/questions/900704/why-does-instance-eval-define-a-class-method-when-called-on-a-class

https://yehudakatz.com/2009/11/15/metaprogramming-in-ruby-its-all-about-the-self/

https://blog.yugui.jp/entry/846