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From: Raffaele Ricciardi <rfflrccrd@gmail.com>
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Subject: How does `undo` know that `keyboard-quit`has been called?
Date: Fri, 07 Nov 2014 22:36:51 +0100
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I would like to write a command that behaves like `undo`, that is: a 
command that reverses its behaviour when the user calls `keyboard-quit`. 
  I have read the Lisp code for both `undo` and `keyboard-quit` and I 
can't understand how `undo` achieves such behaviour. (I know that I 
could always advise `keyboard-quit`.)

Thank you.