UMLR — Section 7.3.44

The metaclasses 'Attribute' and 'AssociationEnd' (defined in UML 1.x) are merged in the metaclass 'Property' in UML 2.x. But the terms 'attribute' and 'association end' are still used in the standard, based one the two possible Property-containments (Property owned by a 'Class' respectively by an 'Association'). In addition, a semantic for the different values of the meta-property 'aggregation' is only defined for a 'Property' of style 'association end'. I propose to re-introduce the metaclasses 'Attribute' and 'AssociationEnd' as specializations of 'Property', making 'Property' an abstract class (the meta-property 'aggregation' should be then moved to AssociationEnd).