A progress bar is not supposed to print one hundred lines just to update its status; generally we like to see a little animation.

一个进度条不应该是打印100行,只是更新它的状态，一般地，我们,想看到一点生机。

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If x is high relative to x-bar--this is positive-- then y tends to be low relative to its mean y-bar and this is negative.

如果x比x均值要大,这个为正,而y比y均值小,这个为负

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