This is the authors draft. Archived with kind permission of Springer Science and Business Media and available at http://www.springerlink.com/content/705882532j178516/fulltext.pdf\ud \ud © Springer-Verlag Berlin Heidelberg 2006The paradigm of contractual specification provides a transparent way of specifying systems. It clearly distinguishes between client and implementer obligations. One of the best known languages used for this purpose is OCL. Nevertheless, OCL does not provide primitives for a compact specification of what remains unchanged when a method is executed. In this paper, problems with specifying invariability are listed and some weaknesses of existing solutions are pointed out. The question\ud of specifying invariability in OCL is studied and a simple but expressive\ud and flexible extension is proposed. It is shown that this extension has a simple OCL based semantics
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