IFML — Fig. 5 - ContentModel references UML Element

The content model (aka Domain Model) should reference a generic modeling element (e.g., DomainElement), so as to enable any kind of domain model to be used with IFML.
UML element can be associated to it.
DomainElement should be extensible.

Created new class in IFML Metamodel: DomainElement. All the classes describing the domain elements (DomainConcept, FeatureConcept, BehavioralFeatureConcept, BehaviorConcept) now extend the DomainElement class, instead of NamedElement. Association from DomainModel is to DomainElements and not to UML::Element.
Fixed Figure 5 by adding the new DomainElement class and its 4 subclasses.
Added new clause describing the DomainElement. Fixed generalization text in the 4 subclasses.