Create a private attribute for the Singleton Pattern called _instance.

为Singleton模式创建私有属性，称为_ instance。

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The self.__last instance attribute (which is a private attribute because it starts with a double underscore) holds the previous mouse click position.

__last实例特性(它是一个私有特性，因为它以双下划线开头)包含上一次鼠标单击位置。

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Some simplistically attribute the decline in our public education system to the drain of skilled students by private schools, but far more significant events were at work.

一些人将公共教育体系的衰退简单粗暴地归因于私立学校技能型学生的流失，但更重要的因素仍在发挥其影响。

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