"Do the clouds make rain? Or is it the rain that makes the clouds? What makes it descend so copiously? Who is it that has the leisure to devote himself, with such abandoned glee, to making these things happen?"
F. Yu, Q. Liu, S. Wu, L. Wang, and T. Tan. Proceedings of the 39th International ACM SIGIR Conference on Research and Development in Information Retrieval, page 729--732. New York, NY, USA, ACM, (2016)