What if you have a "module" within your private code? An example I have is in a pay computation engine. I have a public function that computes how much someone

What if you have a "module" within your private code? An example I have is in a pay computation engine. I have a public function that computes how much someone ought to get paid for a given shift based on who was working the shift, what they were doing and a bunch of other criteria, what day they were working on, what time of day, etc. Within that I have a private function that works a with a narrower scope that just does the pro-rata calculation for time-of-day and day-of-week. That function itself has a fairly high-level interface with a highly complex implementation. IMO it makes sense to test that function, even though that function is private and is only used by the one public function. My point being whether it's private or public isn't really the point. The key thing is whether the functionality is self-contained and decoupled from the rest of the code with a stable interface.