1. Jagiellonian University, Faculty of Mathematics and Computer Science, Theoretical Computer Science Department, ul. Prof. Łojasiewicza 6, 30–348 Kraków, Poland E-mail: bendkowski@tcs.uj.edu.pl; grygiel@tcs.uj.edu.pl
2. University of Lyon, École normale supérieure de Lyon, LIP (UMR 5668 CNRS ENS Lyon UCBL INRIA) 46 allée d’Italie, 69364 Lyon, France. E-mail: pierre.lescanne@ens-lyon.fr
3. Jagiellonian University, Faculty of Mathematics and Computer Science, Theoretical Computer Science Department, ul. Prof. Łojasiewicza 6, 30–348 Kraków, Poland. E-mail: zaionc@tcs.uj.edu.pl