说明：  基于flex和bison的终端计算器代码，如果想要初步编译产生的.c代码能在VC环境下编译的话，请在.l文件的开始加上 option yywrap
(Terminal based on flex and bison calculator code, if you want to produce a preliminary compilation. C code in VC environment compiler, please. L beginning of the file plus option yywrap)