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This will serve as a proof.

以此为凭。

《新英汉大辞典》

Postmark serves as a proof.

邮戳为凭。

《新英汉大辞典》

Even if they do have a proof of identity, none are recognized across the country.

VOA: standard.2009.09.24

That they exist is a proof that American ideals are not the tawdry pretenses they are so often accused of being.

或许这类人的存在,正证明了美国式理想并非无可挑剔,或者说它也在经受着现实的折磨，“

耶鲁公开课 - 1945年后的美国小说课程节选

Of course, that's not the useful application for it, it's more of a proof of principle, but it does show you that you can put it in for studies in any organisms.

当然，这不是很有用的应用，这仅仅是一个原理上的证明，但它告诉你你可以把它,放到研究任何生物体中。

麻省理工公开课 - 化学原理课程节选

Unfortunately, Claystone still feels more akin to a proof-of-concept rather than a fully functional launcher.

ENGADGET: Claystone Launcher 2.0 for Android makes its debut, we go hands-on (video)

The work is also a proof of concept for ProNet, a technology Myriad developed for screening proteins.

FORBES: Magazine Article

But it would offer a way to make better raincoats, and is also a proof of principle.

ECONOMIST: Monitor: Encouraging hydrophobia | The