说明：  本文包含简单中文说明和详细源代码，首先利用Lex进行词法分析，然后用YACC进行语法分析，最后生成执行代码，但是没有对执行代码进行优化。
(This article contains the simple English description and source code in detail, first of all carried out using Lex lexical analysis, and then carried out using YACC syntax analysis, and finally to generate the implementation code, but did not optimize the implementation of the code.)