string: …dialogue system (Curto et al., 2012), sentence transformation (Vanderwende, 2008), machine comprehension (Du et al., 2017; Zhou et al., 2017b; Yuan et al., 2017; Subramanian et al., 2017), question answering (Qin, 2015; Tang et al., 2017; Wang et al., 2017; Song et al., 2017), and visual…