The National Centre for Text Mining, School of Computer Science, The University of Manchester, 131 Princess Street, Manchester M1 7DN, UK. claudiu.mihaila@cs.man.ac.uk|The National Centre for Text Mining, School of Computer Science, The University of Manchester, 131 Princess Street, Manchester M1 7DN, UK. claudiu.mihaila@cs.man.ac.uk|The National Centre for Text Mining, School of Computer Science, The University of Manchester, 131 Princess Street, Manchester M1 7DN, UK. claudiu.mihaila@cs.man.ac.uk|The National Centre for Text Mining, School of Computer Science, The University of Manchester, 131 Princess Street, Manchester M1 7DN, UK. claudiu.mihaila@cs.man.ac.uk