Yeah, that's similar to how I'd look for the correct place to put my tests. But at that point, a unit test is just the innermost layer of tests, which doesn't feel like a useful distinction. In your example, I might have a set of tests checking how the rules are parsed and interpreted (say), and then a set of tests one level up checking that the validation engine was a whole works, and then another set of tests a level up testing a module that uses the validation engine. The tests for the validation engine won't retest parsing, and the tests for the module using the validation engine won't test validation per se, but there's multiple layers there where each layer contains unit tests focusing on that layer's code specifically.