说明：  扫描器类的设计1． 构造算术表达式的四元式翻译文法 2． 设计算术表达式的递归下降子程序分析算法 3． 设计算术表达的四元式生成算法 4． 实现代码并调试运行
(scanner category of a design. Construction arithmetic expressions four yuan grammar-translation 2. Design arithmetic expression analysis subroutine decline recursive algorithm 3. Design arithmetic expression of four yuan-generation algorithm 4. Codes and debugging operation)