通过对PL/0词法分析程序（GETSYM）的分析，并在此基础上按照附录A中给出的PL/0语言的语法描述，编写一个PL/0语言的词法分析程序。此程序应具有如下功能：

输入为字符串（待进行词法分析的源程序），输出为单词串，即由（单词、类别）所组成的二元组序列。

有一定检查错误的能力，例如发现2A这类不能作为单词的字符串。

-By PL/0 lexical analysis program (GETSYM) analysis, PL and on this basis given in accordance with Appendix A/0 language syntax descr iption, write a PL/0 language lexical analysis procedures. This program shall have the following functions:

Enter a string (to be lexical analysis of the source), the output word train, namely by the tuple sequence (word, class) thereof.

There is a certain ability to check for errors, such as a word can not find this kind 2A string.