Advances in Information Retrieval : 34th European Conference on IR Research, ECIR 2012, Barcelona, Spain, April 1-5, 2012, Proceedings - Ricardo Baeza-Yates, Arjen P. Vries, Hugo Zaragoza, B. Barla Cambazoglu, Vanessa Murdock, Ronny Lempel, Fabrizio Silvestri ((Lecture Notes in Computer Science))