PDME — Property-sets for non-Attributable relationships

Summary: Problem: The DocumentRevision (2.7.3.4), DesignSupplier (2.7.3.3) and
PartSupplier (2.7.3.17) relationships inherit from PdmReferencesRelationship.
These relationships have no specified attributes, and
PdmReferencesRelationship is not Attributable. But the factories for
these relationships take a PropertySet parameter. How does the client
subsequently reach any property values so attached?
A similar problem exists with the PartData, PartDataIteration,
PartStructure, PartStructureIteration and PartMasterComposition
relationships (which inherit from PdmContainmentRelationship), but a
previous issue recommends deletion of the factories for Containment
relationships.
A related problem exists for Alternate and Substitute and Usage
relationships, which also inherit from PdmReferencesRelationship
and are not Attributable. In their case, only the properties
defined as "attributes" in the specification are accessible.