说明：  求故障树的 1）对偶树； 2）最小割级和最小路集； 3）假定底事件的故障概率已知，求顶事件发生的概率； 4）求底事件的结构重要度、概率重要度和关键重要度。
(Seeking Fault Tree 1) dual tree 2) minimum cut level and minimal path sets 3) It is assumed that the end of the event of the failure probability known, find the probability of occurrence of the top event 4) Evaluate the end of the event structure importance, important degree of probability and critical degree.)