The research work described in this article has been supported by the National Key Research and Development Program of China under Grant No. 2017YFB1002103. Authors’ addresses: J. Li, H. Li, and X. Kang, Intelligence Building, No. 95, Zhongguancun East Road, Haidian District, Beijing, 100190, China; emails: {junjie.li, haoran.li, xiaomian.kang}@nlpr.ia.ac.cn; H. Yang, NO.152 Luoyu Road, Wuhan, HuBei, 430079, China; email: htyang@mail.ccnu.edu.cn; C. Zong (corresponding author), Intelligence Building, No. 95, Zhongguancun East Road, Haidian District, Beijing, 100190, China; email: cqzong@nlpr.ia.ac.cn. Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. Request permissions from permissions@acm.org. © 2018 Association for Computing Machinery. 2375-4699/2018/11-ART7 $15.00 https://doi.org/10.1145/3234512
