King's College London, Centre for Research on Evolution, Search and Testing (CREST), Department of Computer Science, Strand, London, WC2R 2LS, UK;King's College London, Centre for Research on Evolution, Search and Testing (CREST), Department of Computer Science, Strand, London, WC2R 2LS, UK;Department of Computer Science The University of Sheffield Regent Court, 211 Portobello, Sheffield, S1 4DP, UK;Volgenau School of Information Technology and Engineering, George Mason University, Fairfax, VA 22030, USA;Department of Computer Science, University of York, York YO10 5DD, UK