1. Department of Mathematics and Statistics; McGill University; Burnside Hall Montreal QC H3A 0B9 Canada
2. Department of Computer Science; University of Warwick; Coventry CV4 7AL UK
3. Faculty of Mathematics and Computer Science; Jagiellonian University; Łojasiewicza 6 30-348 Kraków Poland
4. Mathematics Institute, DIMAP and Department of Computer Science; University of Warwick; Coventry CV4 7AL UK