As pointed out by the previous work (Berger and Lafferty, 1999;Gao et al., 2010;Zhou et al., 2011), sentence-query selection requires a distillation of the sentence, while selection of natural language tolerates little being thrown away.
we restrict our attention to those key sentence words that form the distillation of S, do not consider the unaligned words in S, and assume that Q is transfered only from the key sentence words.