I agree with this... If a part of a function is a hotspot of unavoidable complexity then it's good to split it out and test it. But then some people will say t

I agree with this... If a part of a function is a hotspot of unavoidable complexity then it's good to split it out and test it. But then some people will say that you're testing the internals of the caller and that you shouldn't do that. If we follow that logic to the absurd, there should only be one test for the entire application, one for main(), or whatever your entry point is. I've never seen an answer to this problem coded as an intelligible, easy-to-follow guideline. Empirically and intuitively I think my "hotspot" approach kinda works in maximizing reliability.