大多数情况下，我们会在一些代码内部，写一些表达式，在一个脚本里面，也就是Python里面对程序的另一种叫法。
Most of the time, we're going to be doing expressions inside of some piece of code, inside of a script, which is the Python word for program.
你写程序实际上是在写语法树-而其它语言需要编译器生成这种内部表示。
You write programs in the parse trees that get generated within the compiler when other languages are parsed.
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