Make the change easy then make the easy change
The final piece in the mix is arranging that I can focus on the introducing the implementation without getting tangled in the test calibration.
There's a bit of duplication of idea present here; I'm using
the same values on both sides of the test harness to track
which cases are already covered. Don't Repeat Yourself
normally encourages the programmer to remove precisely this
sort of duplication; here, it's just part of the scaffolding:
the duplication will be eliminated by eliminating the scaffolding
altogether.