Advances in information retrieval 43rd European Conference on IR Research, ECIR 2021, virtual event, March 28 - April 1, 2021, proceedings, part II - Hiemstra, Djoerd; Moens, Marie-Francine 1957-; Mothe, Josiane; Perego, Raffaele; Potthast, Martin; Sebastiani, Fabrizio (Lecture notes in computer science, 2021)