Pang, Liang, Yanyan Lan, Jiafeng Guo, Jun Xu, Lixin Su, and Xueqi Cheng. 2019. “HAS-QA: Hierarchical Answer Spans Model for Open-Domain Question Answering”. Proceedings of the AAAI Conference on Artificial Intelligence 33 (01):6875-82. https://doi.org/10.1609/aaai.v33i01.33016875.