A memory is what is left when something happens and does not completely unhappen.

记忆是发生之事，与非完全未发生之事的存留。

youdao

With a slight change of circumstances, we could just as easily unhappen.

NEWYORKER: The Cosmic Menagerie

And still a third is that the Republicans will try to make things that happened over the past two years like health-care reform and financial regulation unhappen, and the resulting government shutdowns and market uncertainty will turn gridlock into a pile-up.

FORBES: Ezra Klein On Gridlock