Conceptual modelling, sometimes in conjunction with requirements acquisition, is widely accepted as the first formal step in information system specification and design. Since knowledge about the world to be modelled is often vague in early development phases, formalisms for conceptual modelling must support a wide variety of concepts for describing real entities. Furthermore a conceptual modelling language must be declarative so that later implementation is not restricted. A large amount of features often hinders learnability and applicability of such languages whereas restriction to few basic features may result in unreadable specifications.