EDIT: Why did I get stuck yesterday? I ended up at a point where I had to plug in a mechnaism that I did not test in isolation, in a system that is untested in isolation. It is good that I've developed some aversion to this. What is still necessary is to formulate a mental model for it as well. The EXT is easy to test. The next step is to make some simpler code that sets up the one-shot.