This is based on this viewpoint: On the one hand, compared with the initial input character sequence, the pruned word lattice has a quite smaller search space while with a high oracle F-measure, which enables us to conduct more precise reranking over this search space to find the best result.
as the structure of the search space is approximately outlined by the topological directed architecture of pruned word lattice, we have a much wider choice for feature selection, which means that we would be able to utilize not only features topologically before the current considering position, just like those depicted in Table 2 in section 4, but also information topologically after it, for example the next word W 1 or the next POS tag T 1 .