R17 - modelint/shlaer-mellor-metamodel GitHub Wiki

R17 / Generalization

Spanning Element is a Lineage, Constrained Loop, Signature, or Activity

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So far, these are the only components of a Domain that flout Subsystem boundaries. There is no hard and fast rule to preclude the discovery of new subclasses. And it may be that Constrained Loop, someday, is subsumed as a special case of constraint which may or may not be Subsystem specific. But, for now, this seems to be the easiest way to manage these Elements.

A Constrained Loop may include Relationship segments from multiple Subsystems. A Lineage may cross Subsystem boundaries since a Subsystem boundary may cross through one or more Generalizations.

Types are considered independent of subsystem as their usage tends to be scattered around everywhere.

As for Signatures and Activities, we just number them uniquely within a Domain without regard to any Subsystem.