[josephg]

I mean, yeah, I thought that was obvious. If you want to be a pedant:

> Tests only pass if both implementations of your software behave the same way in the exact area being tested.

As I said in my comment above. Tests are a crappy second implementation. The test in your example isn’t even defined outside the input range of (2,2). Tests are a stochastic tool. Tests can prove the presence of a bug, not their absence. Completeness isn’t something tests alone can provide. But in the choice between yolo coding and yolo coding plus tests, you’re obviously going to get fewer bugs with tests.