The example given with the `Point` struct is interesting. As a self taught developer with admittedly little language design knowledge, it almost seems like unit tests without all of the boiler plate.

For instance, the `is_diagonal` function provided as the "correctness checker" looks to me like something that would be in a unit test for the `double` function. However, I think it goes a little bit deeper than that. In a unit test, you are somewhat required to pass specific values to the constructs you are testing. The given example is more complete than a unit test in that it doesn't just test for the correct value being produced by the `double` function when given specific values, it actually checks all possible outcomes by asserting that the `double` function always produces a `Point` where `p.x == p.y`.

Given that the example is relatively trivial, I would like to see how this would work for more complex functions and structures.