Institute of Informatics, Warsaw University, Banacha 2, 02-097 Warszawa, Poland. dojer@mimuw.edu.pl|Institute of Informatics, Warsaw University, Banacha 2, 02-097 Warszawa, Poland. dojer@mimuw.edu.pl|Institute of Informatics, Warsaw University, Banacha 2, 02-097 Warszawa, Poland. dojer@mimuw.edu.pl|Institute of Informatics, Warsaw University, Banacha 2, 02-097 Warszawa, Poland. dojer@mimuw.edu.pl|Institute of Informatics, Warsaw University, Banacha 2, 02-097 Warszawa, Poland. dojer@mimuw.edu.pl