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The purpose of visitor mode is to separate processing from data structure. Many systems can be separated according to algorithm and data structure. If such systems have relatively stable data structure, they are easy to　　为了保住村民的救命粮食，武僧决定与山贼一战，由此展开了一场秋收保卫战。"Clean and jerk snatch narrow pull high turn down squat turn over clean and jerk use force to push high grasp support squat overhang turn" video collection1.2 Flammability of fabrics为了引起家人的注意，一个过度劳累的母亲，向他们谎报自己的病情。他一指万婆子，神情十分得意：有人证，还是你们村的，他怕什么？不等泥鳅说话，就听门口有人骂道：你这恶毒的老贼婆，你恩将仇报，死后要下地狱的。The principle of opening and closing means opening to expansion and closing to modification. When the program needs to be expanded, the original code cannot be modified, but the original code should be expanded to achieve a hot plug effect. Therefore, a summary of one sentence is: in order to make the program scalable, easy to maintain and upgrade. To achieve this effect, we need to use interfaces and abstract classes, etc. We will mention this point in the later specific design.