Multilevel modeling: What's in a level? A position paper

Multi-Level Modeling (MLM) conceptualizes software models as layered architectures of sub-models whose elements are inter-related by the instance-of relation, which is radically different from the traditional two level representations that consist of a class model and an instance model, where the former represents the schema and the latter the population. Key to the MLM representation is the notion of a level, which is the subject of this position paper. We argue that a theory of MLM must have a distinguished notion of level, which should have both a syntactic and a semantic status. Moreover, the notion of a level must represent a class model, and the overall MLM theory must be compositional, being composed from the theory of class models.