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"To bed, to bed. There's a knocking at the/gate. What's done cannot be undone. To bed, to/bed, to bed"
This is said when Lady Macbeth is sleepwalking and her conscience is getting to her as she is haunted with the murder of Duncan. Before she said what's done is done but now she is saying it can't be undone and that there is nothing she can do to change what happened. Lastly, to bed can symbolize three things; after Macbeth and her murdered Duncan they went to bed, her maritial problems and maybe rape and her deathbed.