Comment by mrkeen

Off the top of my head:

If you're using the product, and you want to question or debug what's going on, you can:

* Jump directly to the single relevant part of the frontend responsible

* Likewise with the backend. The layout and naming of the code should scream its purpose.

* Once you're looking at the code, it should be trivial to run it, right now, instantly, in unit test, or cli. You shouldn't need to stand up a database to see whether your code rounds taxes the expected way.

The system contains its own checkability. You can, for instance, just sum up all the incoming money and outgoing money and see if your balance is correct. (It's not enough to have good tests today, if you're working on data that was incorrectly calculated and stored yesterday)