在别的语言里，有一个叫做“别名”。
但不用别名也行，就是为啥不使用“一个”变量？而要用“两个”？

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可以建立一个类，这个类包含两个变量。对这两个变量的其中任何一个变量的赋值的修改，都会由类中的程序自动应用于另外一个变量。