C D S (Koehn et al., 2003) • sequence (Galley and Manning, 2010) • • sequence  and • tree  This work • • graph Table 1: Comparison between our work and previous work in terms of three aspects: keeping continuous phrases (C), allowing discontinuous phrases (D), and input structures (S).
the expressiveness of these models is confined by hierarchical constraints of the grammars used (Galley and Manning, 2010) since these patterns still cover continuous spans of an input sentence.