Research group PLIS: Programming, Logic and Intelligent Systems, Department of Communication, Business and Information Technologies, Roskilde University, P.O. Box 260, Roskilde, DK-4000, Denmark. cth@ruc.dk|Research group PLIS: Programming, Logic and Intelligent Systems, Department of Communication, Business and Information Technologies, Roskilde University, P.O. Box 260, Roskilde, DK-4000, Denmark. cth@ruc.dk|Research group PLIS: Programming, Logic and Intelligent Systems, Department of Communication, Business and Information Technologies, Roskilde University, P.O. Box 260, Roskilde, DK-4000, Denmark. cth@ruc.dk