我们需要对v.d进行详细观察！ Lemma 24.11 (Upper-bound property) Let G =(V,E)be a weighted,directed graph with weight function w E->R. Let s e V be the source vertex,and let the graph be initialized by INITIALIZE- SINGLE-SOURCE(G,s).Then,v.d >8(s,v)for all vV,and this invariant is maintained over any sequence of relaxation steps on the edges of G.Moreover, once v.d achieves its lower bound 8(s,v),it never changes. 计算中不断变化（其实是不断“变小”） 的v.d始终是最短距离的上界！我们需要对v.d进行详细观察！ 计算中不断变化(其实是不断“变小”) 的v.d始终是最短距离的上界！