Units of measure — Classes, and the units inside them

A class is a dimension — mass, length, count, volume, time, area, or anything your business invents. Inside it sit the units: mm, m, km. One unit in each class is the base, and every other unit says how many base units it is worth (a millimetre against a base of metres is 0.001). A workspace arrives with a seeded library so nobody starts from an empty page. Seeded rows are editable but flagged, so "did I define this, or did RowFold?" always has an answer.