索引的原文档(Document)。
为了方便说明索引创建过程，这里特意用两个文件为例：
文件一：Students should be allowed to go out with their friends, but not allowed to drink beer.
文件二：My friend Jerry went to school to see his students but found them drunk which is not allowed.
结果处的索引文件：
- Document Frequency 即文档频率，表示总共有多少篇文档包含此词(Term)。
- Frequency 即词频率，表示每篇文档里面包含了几个词(Term)。
- 左边是词典，右边是倒排表