Abstract. The Meta-Modeling Language is a static object-oriented modeling language whose focus is the declarative definition of languages. It aims to enable the UML metamodel to be precisely defined, and to enable UML to evolve into a family of languages. This paper argues that although MML takes a metamodeling approach to language definition, it cannot be described as strict metamodeling. This has significant implications for the nature of the metamodel architecture it supports, yet without contravening the OMG's requirements for the UML 2.0 infrastructure. In particular it supports a rich generic nested architecture as opposed to the linear architecture that strict metamodeling imposes. In this nested architecture, the transformation of any model between its representations at two adjacent metalevels can be described by an information preserving one-to-one mapping. This mapping, which can itself be defined in UML, provides the basis for a powerful area of functionality that any potential metamodeling tool should seek to exploit.

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A key aspect of successfully using UML is understanding the semantics of the notations. UML 2 will increase the already substantial collection of notations supported by UML 1.x. At the same time, this will augment the difficulty users experience in understanding semantics. In this paper we propose that while the diverse notations may render concepts differently, the concepts can often be considered semantically equivalent. This gives rise to an architecture where two single abstract syntaxes (structure and behaviour) underpin UML 2's seven concrete syntax. Because there a fewer semantically distinct concepts, this makes UML both easier to understand and substantially easier to implement.

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