It helps to reduce search space because all key information of X for task completion has been included in B t .
to recent work (Eric and Manning, 2017a) that also employs a copy-attention mechanism to generate a knowledge-base search API and machine responses, our proposed method advances in two aspects: on one hand, bspans reduce the search space from U 1 R 1 ...U t R t to B t−1 R t−1 U t by compressing key points for the task completion given past dialogues; on the other hand, because bspans revisit context by only handling the B t with a fixed length, the time complexity of TSCP is only O(T ), comparing O(T 2 ) in (Eric and Manning, 2017a).