Which One Is the Source Code?

A rule you write once usually ends up copied into your server, your web page, your database and your documentation, and the copies drift. Starting from an empty folder, we put one greeting rule into seven generated files at once, lined up so the rule reads in a single column, with the rulebook along the bottom. Changing a name moves no code at all; changing the rule moves every file as one set, and a small mobile app that runs the generated TypeScript follows along. Then the twist: we change the rule in Excel and bring it home with xlsx-to-rulebook, and finally just ask Claude for it. The rulebook is the single source of truth, a role it holds, not a privileged file.