Faculty of Mathematics and Computer Science, Friedrich-Schiller-University Jena, 07743 Jena, Germany. thasso@minet.uni-jena.de|Faculty of Mathematics and Computer Science, Friedrich-Schiller-University Jena, 07743 Jena, Germany. thasso@minet.uni-jena.de|Faculty of Mathematics and Computer Science, Friedrich-Schiller-University Jena, 07743 Jena, Germany. thasso@minet.uni-jena.de