[1] De Montfort Univ, Fac Comp Engn & Media, Leicester LE1 9BH, England
[2] Leiden Inst Adv Comp Sci, Fac Sci, Niels Bohrweg 1, NL-2333 CA Leiden, Netherlands
[3] Univ Jyvaskyla, Fac Informat Technol, POB 35 Agora, FI-40014 Jyvaskyla, Finland
[4] Univ Innsbruck, Fac Engn Sci, Dept Infrastructure Engn, Unit Environm Engn, Technikerstr 13, A-6020 Innsbruck, Austria