Units of measure — Refused, never guessed

Every unit belongs to a class — a kind of thing being measured. Converting inside a class is exact arithmetic. Converting across one is not arithmetic at all, so RowFold declines: • Across classes, conversion is refused. Kilograms to metres depends entirely on what is being measured, and there is no factor that holds for every record. A silent 1:1 fallback is precisely how a number ends up wrong by three orders of magnitude with nothing raised. • A unit the workspace does not have is refused — “'stone' isn't a unit in this workspace.” — rather than being dropped so the bare number lands. • A column that only takes its own unit says so — “'Weight' only takes kg.”