I view unit tests as a kind of proof by counter-example. You have a logical structure your program embodies. This structure is very hard to specify mathematical

I view unit tests as a kind of proof by counter-example. You have a logical structure your program embodies. This structure is very hard to specify mathematically and prove deductively so you come up with key statements that must at least evaluate to true for this structure/theory. The tests are a bunch of counter-examples that should be false (test passed). If a random testing framework is available in your language they really should be integrated as they are able to come up with some pathological examples.