编译原理实现：实现基本的递归下降分析器，能够分析任给的符号串是否为该文法所定义的合法算术表达式。实验报告中要说明分析使用的方法。-Compiler theory to achieve: to achieve the basic recursive descent parser, to analyze whether any given string of symbols for the grammar defined by legal arithmetic expressions. Experimental analysis of the report to show the methods used.