The Unified Modeling Language (UML) still lacks a formal and commonly agreed specification of its semantics that also accounts for UML's built-in semantic variation points and extension mechanisms. The semantics specification of such extensions must be formally integrated and consistent with the standard UML semantics without changing the latter. Feasible semantics approaches must thus allow advanced UML modelers to define domain-specific language extensions in a precise, yet usable manner. We

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... ave proposed dynamic meta modeling for specifying operational semantics of UML behavioral diagrams based on UML collaboration diagrams that are interpreted as graph transformation rules. Herein we show how this approach can be advanced to specify the semantics of UML extensions. As a case study we specify the operational semantics of UML sequence diagrams and extend this specification to include features for modeling multimedia applications.