Intelligent Systems Laboratory and Advanced Database Research and Modelling, Department of Mathematics and Computer Science, Middelheimlaan 1, B-2020 Antwerpen, Belgium.|Intelligent Systems Laboratory and Advanced Database Research and Modelling, Department of Mathematics and Computer Science, Middelheimlaan 1, B-2020 Antwerpen, Belgium.|Intelligent Systems Laboratory and Advanced Database Research and Modelling, Department of Mathematics and Computer Science, Middelheimlaan 1, B-2020 Antwerpen, Belgium.|Intelligent Systems Laboratory and Advanced Database Research and Modelling, Department of Mathematics and Computer Science, Middelheimlaan 1, B-2020 Antwerpen, Belgium.