¨ochst¨adtplatz 5, 1200 Vienna, Austria3Institute of Computer Technology, Vienna University of Technology, Gußhausstraße 27-29/E384, 1040 Vienna, Austria4Oregano Systems –
Design and Consulting GesmbH, Phorusgasse 8, 1040 Vienna, AustriaReceived 1 May 2006; Revised 21 S eptember 2006; Accepte[r]