Typically, trees used in SMT are either phrasal structures (Galley et al., 2004;Marcu et al., 2006) or dependency structures Xiong et al., 2007;Xie et al., 2011;Li et al., 2014).
note that we treat the adjacent previous subgraph differently from others since information from the last previous unit is quite useful (Xiong et al., 2006;Cherry, 2013).