Encapsulating instance state us not the point, encapsulating the class behaviour is the point. So, if you can make the prototype itself private and modify the p

Encapsulating instance state us not the point, encapsulating the class behaviour is the point. So, if you can make the prototype itself private and modify the prototype with class methods, you have something that prototypes by themselves do not express. The analogy is not to a private instance variable in Ruby, the analogy is to a class method in Ruby Dutch as define_method.