Affiliation: | Z Department of Statistics, University of Leeds, Leeds LS2 9JT, UK E-mail: j.t.kent@leeds.ac.uk ZZ School of Mathematical Sciences, University of Nottingham, Nottingham NG7 2RD, UK E-mail: ild@maths.nott.ac.uk Y Garvin and Company, Redditch B97 4BS, UK |