Primitive types in the ivoa package

[Laurent MICHEL]

Helo DM,

I'm working on CAB-MSD and Modelio warns me against the use of 
ivoa:Quantity as attribute types because it is not declared as primitive.

    For the record: ivoa:Quantity and sub-types attach a unit
                    to a numerical value.

This means that I've to build an explicit composition relationship from 
my class to ivoa:Quantity if I want to use it which is rather ugly.

Is there is strong justification for this feature?

Laurent
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