ten restricted to government documents or news articles.
improving NMT under small-data training conditions has attracted extensive attention in recent years (Sennrich et al., 2016a;Cheng et al., 2016;Zoph et al., 2016;Chen et al., 2017;Fadaee et al., 2017;Ren et al., 2018;Lample et al., 2018).