生成鼠标键按下和释放事件。

Generating mouse button press and release events.

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如果在Tk鼠标被按下Submit之后立即提取事件，cnee程序将不会提取鼠标按下事件(及后续释放事件)。

If events are extracted immediately after the Tk mouse is pressed on Submit, the mouse-down event (and subsequent release event) will not be picked up by the cnee program.

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根据程序的状态，鼠标按下事件，应该开始绘制一条直线，或者选中一个对象。

Depending on the state of the program, a mouse down might start drawing a line, or it might select an object.

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