Convertible Units

I can define a Convertible Unit: For example, "Quart" or "Kilometer" or "Ratl".

I can define a Convertible Unit Category: That is, I can define a category such as "Volume", "Length", "Speed", and so on.

I can define a Convertible Unit System: A system is something like "Metric", "Modern English", "Medieval English", "Arabic", etc. It is a collection of Convertible Units.

I can define a Property of a specific Unit Category: That is, I can say that a Property is of type Volume. Any Volume from any System can be used.

I can define a Property of generic Unit Category: 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.

I can define the quantity relationship between Units of the same Category: That is, I can define that 1 Quart is 4 Cups, or that 1 Mile is 1.60934 Kilometers.

I can define the relationship between Units of different Categories: This is where we get really fancy, defining rules that teach the system, eg, that the Speed Category can be defined in Distance / Time, and that "miles per hour" is a standard unit of Speed in the English System.