Guest editorial to the theme section on multi-level modeling

Multi-level modeling (MLM) [5] represents a significant extension to the traditional two-level object-oriented paradigm with the potential to improve upon the utility, reliability, and complexity of models. Different from conventional approaches, MLM approaches allow for an arbitrary number of classification levels and introduce other concepts that foster expressiveness, reuse, and adaptability. A key aspect of the MLM paradigm is the use of entities (so-called clabjects) that are simultaneously types and instances [6], a feature which has consequences for conceptual modeling, for language engineering, and for the model-based development of software-intensive systems. MLM facilitates also deep instantiation [7], which, in contrast to shallow instantiation, allows model elements at a level to not only specify a scheme for elements at the next lower level but also to specify schemes for elements located at levels further down in the hierarchy. Different MLM approaches use different techniques to control and maintain this kind of instantiation. In Potency-based approaches [6, 8], for instance, a natural number (potency) is assigned to each model element indicating how many levels down in the hierarchy that element can be instantiated. Different variants of potency have been proposed to satisfy practical requirements, such as leap potency (facilitating jumps over levels) and depth (enforcing the last level at which an element may be instantiated).