The reasons
The most remarkable chapter of the 1446 book is the one that explains why: why ㄱ (g) is shaped like the back of the tongue blocking the throat, why the vowels build from three primitives. That chapter was lost for five hundred years, so the world had the letters but not the reasons. A surviving copy came to light in 1940. The letters worked the whole time. But a system whose reasons are lost cannot be extended, only used.
This part is gog’s reasons, published with the package on purpose. It states the nine laws the package never breaks, and the one shortcut they all exist to refuse. It says why the engine refuses what it refuses, and why every refusal comes with a direction. None of it is required to use gog. The earlier parts cite these laws by name, and never ask you to have read them. It is here for the moment you want to know why the grammar is shaped the way it is. It also predicts what the engine will do in a case no chapter covers.
The laws all refuse the same thing, and it has a name: the expert’s shortcut, the saved keystroke that takes a rule away from the beginner.