voidadj.空的；无效的；缺乏的n.真空；空白；空虚vt.使无效；排泄；使空出vi.排泄Void - free无空洞voids content孔隙含量void swelling辐照肿胀void detection空隙检测voiding volume尿量void and solid虚实处理voidage factor亏空因子voided bladder排尿囊(膀胱)voidable trust可撤销的信托voided sleeper轨枕悬空

1. The null set is any set subset.
   空集是指不含任何元素的集合，空集是任何集合的子集。