Rails 3 modularity

I was reading through ActionDispatch::Routing::Mapper to understand how the routing DSL was implemented and ended up learning a really nice technique for modularizing a class. Here is the gist of it:

Probably not something you want to do all the time. For framework code, where you want to provide building blocks that people can pull out bits and pieces of functionality, or swap out pieces with their own, this is pretty much how rails 3 does it.