I can define a Property of generic Unit Category

(User Story, Investigate , Priority: Low, Test Status: No automated tests yet , Reported By Justin du Coeur, )
Convertible Units
Summary: This is tricky, but is one of the motivating cases. In the Cooks' Guild Ingredient list, I want to be able to say that an Ingredient has a "quantity", which might be a Count, it might be a Volume, or it might be entirely empty.
This will need design, to figure out exactly what it means, but it is highly desireable.