UML is currently embraced as "the" standard in object-oriented modeling languages, the recent work of OMG on the Meta Object Facility (MOF) being the most noteworthy example. We welcome these standardisation efforts, yet warn against the tendency to use UML as the panacea for all exchange standards. In particular, we argue that UML is not sufficient to serve as a tool-interoperability standard for integrating round-trip engineering tools, because one is forced to rely on UML's built-in extension mechanisms to adequately model the reality in source-code. Consequently, we propose an alternative metamodel (named FAMIX), which serves as the tool interoperability standard within the FAMOOS project and which includes a number of constructive suggestions that we hope will influence future releases of the UML and MOF standards.