[_fbpt]

Fascinating idea. I've definitely written and encountered programs where functions were more complicated than they could've been, in order to support features separate from the "main idea" of the program. Like FPS calculation, Ctrl+C handling, passing intermediate audio data to extra buffers for visualization...

Could "splicing in" code it be implemented using aspect-oriented programming frameworks? https://en.wikipedia.org/wiki/Aspect-oriented_programming I've never used them though.

> There's a new constraint that has no counterpart in current programming practice — remove a feature, and everything before it should build and pass its tests: > build_and_test_until

Looks like added tooling is needed, but seems worthwhile.