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Association is a Unary Association or Binary Association

An Association describes one or two named Perspectives on a mapping among class instances.

Each Perspective establishes a role relationship on each side of the mapping.

The unary/binary distinction refers to the number of Perspectives and not the number of Classes involved. Thus a Unary Association is another name for a Symmetric Reflexive Association as described in Section 4.2 of [OOA96].

A Binary Association may establish a mapping among instances of the same class (reflexive binary) or between instances of two different classes.

<subclass>.(Rnum, Domain) -> Relationship.(Rnum, Domain)