Deriving Identity from Extensionality
In recent years, a number of proposals have been made to extend conventional conceptual data modeling techniques with concepts for modeling complex object structures. Among the most prominent proposed concepts is the concept of collection type. A collection type is an object type of which the instances are sets of instances of another object type. A drawback of the introduction of such a new concept is that the formal definition of the technique involved becomes considerably more complex. In this paper a new kind of constraint, the extensional uniqueness constraint, is introduced, providing an alternative treatment of complex object types. The formal definition of this constraint type is presented, the advantages of its introduction are discussed, and its consequences for, among others, identification schemes are elaborated.