Department of Computing and Information Systems, The University of Melbourne, Parkville, 3010, VIC, Australia. antonio.jimeno@gmail.com.|UNED NLP & IR Group, C/Juan del Rosal 16, Madrid, 28040, Spain. lplaza@lsi.uned.es.|UNED NLP & IR Group, C/Juan del Rosal 16, Madrid, 28040, Spain. jcalbornoz@lsi.uned.es.|National Library of Medicine, 8600 Rockville Pike, Bethesda, MD 20894, USA. mork@nlm.nih.gov.|National Library of Medicine, 8600 Rockville Pike, Bethesda, MD 20894, USA. alan@nlm.nih.gov.