Modeling Language Variability
  A systematic way of defining variants of a modeling language is useful for adopting the language to domain or project specific needs. Variants can be obtained by adopting the syntax or semantics of the language. In this paper, we take a formal approach to define modeling language variability and show how this helps to reason about language variants, models, and their semantics formally. We introduce the notion of semantic language refinement meaning that one semantics variant is implied by another. 
Hans Gr\"onninger, Bernhard Rumpe
cs.SE