在Pil中，chunk的定义：
Each piece of code that Lua executes, such as a file or a single line in interactive mode, is called a chunk. A chunk is simply a sequence of commands (or statements).
Lua执行的一段代码，无论是一个Lua文件，或者仅仅是一条语句，均统称为chunk。说白了，chunk就是一段语句（statement）。