It would work if you somehow could get the value of `z` (the "return" value of `add` in the example). Then you have a unit test for `add` on its own, and if you

It would work if you somehow could get the value of `z` (the "return" value of `add` in the example). Then you have a unit test for `add` on its own, and if you also unit test `f1`, `f2` and `f3` (which it calls depending on the value of `z`), you'd have almost full unit test coverage. (You're not testing the conditions that lead to calling one of the `f` functions. Maybe it's chalked up to integration testing?) But yeah, I share much of your frustration. The Decision making section should be the first thing, and it should have a test example. Longer non-trivial programs examples should be contrived to reassure people that this not utterly impractical.