^aBRICS (Basic Research in Computer Science), Centre of the Danish National Research Foundation, Department of Computer Science, Aalborg University, Fr. Bajersvej 7B, 9220 Aalborg Ø, Denmark ^bDepartment of Computer Science, School of Science and Engineering, Reykjavik University, Ofanleiti 2, 103 Reykjavik, Iceland ^cDepartment of Computer Science, University of Bologna, Mura Anteo Zamboni 7, 40127 Bologna, Italy ^dMicrosoft Research, Roger Needham Building, 7 J.J. Thomson Avenue, Cambridge CB3 0FB, United Kingdom ^eVrije Universiteit Amsterdam, Department of Computer Science, Section Theoretical Computer Science, De Boelelaan 1081a, 1081 HV Amsterdam, The Netherlands