What does this get you that you don't get by putting an unexported method in your Go interface today? Honest question, since I'm not 100% sure I'm sharing the t

What does this get you that you don't get by putting an unexported method in your Go interface today? Honest question, since I'm not 100% sure I'm sharing the terminology of the author, and I don't know if this is a matter of the author being unaware of this possibility, or aware and not satisfied for some reason I don't understand. You don't get a literal enumeration of all implemented types in the interface specification itself, but it's still trivial/mechanical to extract all implementations of such an interface with some code tool, and no external package can add to the list. (Since I've learned from experience this always comes up: If you put a method named 'unexported' on your interface, it doesn't matter if a value in some other package creates a method called 'unexported', the compiler will not consider it a match. An interface in some package with an unexported method can not be implemented by any other package.)