基类event描述时间、地点、事件；meeting类描述时间、地点、会议议程、参会人员；lecture类描述时间、教室地点、授课教师、课程名称、授课内容。(The base class event describes the time, place, and event; the meeting class describes the time, place, meeting agenda, participants, class lecture, descr iption time, clas