1. Centrum Wiskunde and Informatica, Department of Software Engineering, Amsterdam, The Netherlands. F.S.de.Boer@cwi.nl
2. Department of Computer Science and Engineering / Focus Team INRIA, University of Bologna, Bologna, Italy. mario.bravetti@unibo.it
3. Faculty of Mathematics, Astronomy and Physics (FAMAF), National University of Córdoba, Córdoba, Argentina. lee@famaf.unc.edu.ar
4. Department of Computer Science and Engineering / Focus Team INRIA, University of Bologna, Bologna, Italy. gianluigi.zavattaro@unibo.it