The model assumes each source word is assigned to exactly one target word, and defines an asymmetric alignment for the sentence pair as 1 = 1 , 2 , … , , … , , where each ∈ [0, ] is an alignment from the source position j to the target position , = 0 means is not aligned with any target words.
in our model, the word distance is calculated based on the previous node in BUTorder rather than the previous word in the original sentence.