Department of Mathematics, University of Oslo, Moltke Moes vei 35, Oslo, 0851, Norway. knutdr@math.uio.no.|Department of informatics, University of Oslo, Gaustadalleen 23 B, Oslo, 0371, Norway.|Department of informatics, University of Oslo, Gaustadalleen 23 B, Oslo, 0371, Norway.|Department of Mathematics, University of Oslo, Moltke Moes vei 35, Oslo, 0851, Norway.|Department of Mathematics, University of Oslo, Moltke Moes vei 35, Oslo, 0851, Norway.|Department of informatics, University of Oslo, Gaustadalleen 23 B, Oslo, 0371, Norway.