unified file

• 统一文件

unified file的用法和样例：

例句

1. The version function described later in this chapter shows how a file-centric vision of Undo can be implemented so that it works well with the unified file model.
   本章后面要讨论的生成多个版本的功能是一种文件级的撤销方式，这样它们能够在统一文件模型下工作。