1. Department of Computer Science, University of Rome I, Italy. carlucci@di.uniroma1.it
2. Institute of Mathematics, University of Warsaw, Poland. lak@mimuw.edu.pl
3. Department of Computer Science, University of Rome I, Italy. lepore90@gmail.com
4. Faculty of Mathematics and Natural Sciences, Cardinal Stefan Wyszyński University in Warsaw, Poland. k.zdanowski@uksw.edu.pl