[1] Kings Coll London, CREST, Dept Comp Sci, London WC2R 2LS, England
[2] Univ Sheffield Regent Court, Dept Comp Sci, Sheffield S1 4DP, S Yorkshire, England
[3] George Mason Univ, Volgenau Sch Informat Technol & Engn, Fairfax, VA 22030 USA
[4] Univ York, Dept Comp Sci, York YO10 5DD, N Yorkshire, England