How does if-not-modified-since work?

if-not-modified-since的工作原理是什么?

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Last week Cisco chief executive John Chambers said his company was likely to invest more overseas if US tax laws were not modified.

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The anthropogenic origins of climate change can therefore be modified, if not reversed, if humans can have a better understanding of the consequences of their ethos on the environment.

UNESCO: THEMES

That, to some Zapatists, suggests that he will not mind if the bill is blocked or modified.

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