the collections classes in ruby, array hash, etc. all use the enumerable module which can be included in your classes, just like Comparable, as long as you define an each method that returns every object of your collection in turn.
defin a each method and then what it yeilds, and this will be put in the enumerable calls
another example where you create a module and then include it, this time in the Array and Range classes which are already in Ruby.  Performs the function you define.
can assign a module to a hash of the state id so that the method doesn't clash.
end of chapter five, really cool discussion about difference between inheritance and mixins - now i understand mixins, they are module includes - and why to use them - better flexibility than inheritance and more maintainable code.