To keep track of which variables can be used where, it is sometimes useful to draw a stack diagram.

要追踪一个变量能在哪些位置使用，咱们就可以画个图表来实现，这种图表叫做栈图。

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Pop the stack and draw a green rectangle to remove the previous translation and scaling.

从堆栈中取出坐标，绘制一个绿色矩形，以便删除之前的转换和缩放。

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Pop the stack and draw a green rectangle to remove the previous translation and scaling.

从堆栈中取出坐标，绘制一个绿色矩形，以便删除之前的转换和缩放。

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