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You cannot undo what has already been done.

I can't undo it.

The "undo" option reverts the last change done to the document.

He did not want to know, but you cannot undo knowledge.

David's mother had been developing a pill to undo the effects of the operation.

Could you help me undo the buttons?

Please undo the package.

Don't undo your bootlaces until you have seen the river.