Pang, L., Y. Lan, J. Guo, J. Xu, L. Su, and X. Cheng. “HAS-QA: Hierarchical Answer Spans Model for Open-Domain Question Answering”. Proceedings of the AAAI Conference on Artificial Intelligence, vol. 33, no. 01, July 2019, pp. 6875-82, doi:10.1609/aaai.v33i01.33016875.