@techreport{MuellerPoetzsch-HeffterLeavens04a, author = {M\"{u}ller, P. and Poetzsch-Heffter, A. and Leavens, G. T.}, title = {Modular Invariants for Layered Object Structures}, institution = {Department of Computer Science, ETH Zurich}, year = {2004}, number = {424}, urltopdf = {http://people.inf.ethz.ch/lehnerh/pm/publications/getpdf.php?bibname=Own&id=MuellerPoetzsch-HeffterLeavens04a.pdf} }