> You can also update the definition of a class and then force all instances of this class to use the updated definiton... all of this while the program keeps r

> You can also update the definition of a class and then force all instances of this class to use the updated definiton... all of this while the program keeps running. Right. In fact, you can define a method to be run on instances that were created before the class was redefined, that updates them for the new definition. So if you add a slot to the class, and you need it to be initialized in some way, you can define this method to do what you need.