[1] Univ Manchester, Dept Comp Sci, Manchester M13 9PL, Lancs, England
[2] Bar Ilan Univ, Dept Comp Sci, IL-52900 Ramat Gan, Israel
[3] Kings Coll London, Dept Informat Sci, London WC2R 2LS, England
[4] Fac Sci, Comp Sci & Commun, Luxembourg, Luxembourg
[5] Univ British Columbia, Dept Philosophy, Vancouver, BC, Canada