Department of Computer Science, Royal Holloway, University of London, Egham, TW200EX, UK, School of Mathematics, Statistics & Applied Mathematics, National University of Ireland, Galway, School of Biological Sciences, Royal Holloway, University of London, Egham, TW200EX, UK and Dipartimento di Informatica, Università degli Studi di Milano, via Comelico 39/41, 20135 Milano, Italy.