I'm not saying you cannot do that (and, specifically, I am not saying you cannot do the sort of things the author seems to want to do.) I am saying that I don't

I'm not saying you cannot do that (and, specifically, I am not saying you cannot do the sort of things the author seems to want to do.) I am saying that I don't see how you can write the program in the first place, unless you have some semantics in mind for every identifier you introduce. To add to that, I doubt to can perform a runtime substitution unless the substitution conforms to the semantics of the original design. This seems to be the case with the author's examples, which extend the original types with a sidechannel for getting information about the execution of the program.