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From: Richard Stallman <rms@gnu.org>
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Subject: Re: [rant] Should reverting a buffer really discard undo history?
Date: Wed, 01 Jun 2005 13:25:33 -0400
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    I'm not real familiar with the mechanics of the undo list, but couldn't the
    previous file contents be stored only on disk, as a temporary file, and the
    value in the undo list be just a pointer to that file instead of the file
    contents?

It would be conceivable to store old undo-list contents in files.  15
years ago I wanted to implement that in Emacs.  Nowadays it seems less
important, because memory is big enough to hold more undo list data
than in the past.  It's only for something like this that one would
think of wanting to do that.