It isn't complicated compared to the convoluted stuff that programmers code to implement business "logic." The dirty secret of software development, however, is that 90% of industrial software developers can't handle abstraction at all and rely heavily on domain knowledge when coding anything. Programmers handle the cases that are normal for the problem domain, and QA tests the cases that are unusual in the domain context (and makes the programmer fix them.)

The code will behave incorrectly on some edge cases, but if the developers and QA have covered the problem domain pretty well, it won't matter. First, most of the edge cases will never happen. Second, people get angry when software fails for no obvious reason, but they tend to be a lot more forgiving when software fails on inputs that are "freakish" or "wrong" in the domain context. Searching the widget catalog results in "500 Internal server error" when the widget catalog is empty? Well, duh! When should the widget catalog ever be empty?! The search page shows screwy results because someone entered an item with a negative price? Well, duh! Get that stupid item out of the catalog! Instead of thinking like a mathematician or a system designer in order to avoid mistakes, the typical development approach is to think like the customer so you only make mistakes that the customer can sympathize with.