“…As for tree-based NMT models, a lot of different methods have been proposed. Trees can be used in either the source side (Li, Xiong, Tu, Zhu, Zhang, and Zhou 2017) or the target side (Aharoni and Goldberg 2017), or both (Wu et al 2018), can be encoded either using treestructured neural networks (Eriguchi et al 2016) or with the help of linearization (Sennrich and Haddow 2016), and can be either constituent trees (Chen, Huang, Chiang, and Chen 2017) or dependency trees (Wu, Zhang, Yang, Li, and Zhou 2017). As for forest-based NMT models, Ma et al 2018is the first attempt, where linearized packed forests are encoded using RNNs in order to make the model robust to parsing errors.…”