UMLR — ptc-03-09-15/Explain the new association modeling constructs

Issue: The Core’s abstract syntax makes heavy use of new association
modeling constructs. Two of them in particular may be unfamiliar to many
who read the submission:
· Subsets
· Derived unions

The submission provides only a brief explanation of these two new
constructs, which I quote below:

"A navigable property may be marked as a subset of another, as long as the
owner of the subsetting property is the same as or a specialization of the
owner of the subsetted property. In this case, the collection associated
with an
instance of the subsetting property must be included in (or the same as) the
collection associated with the corresponding instance of the subsetted
property.

A property may be marked as being a derived union. This means that the
collection of values denoted by the property in some context is derived by
being the strict union of all of the values denoted, in the same context, by
properties defined to subset it. If the property has a multiplicity upper
bound of 1, then this means that the values of
all the subsets must be null or the same."

Recommendation: Since these constructs are so heavily used to define the
Core itself, it would be useful for the submission to provide some overall
guidance on how to use them. Providing rationale for why specific ones are
chosen in specific places in the definition of the Core would be an
effective way of disseminating understanding of these constructs and
understanding of the Core as well.