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The debugger keeps a history of your interactions that modify the state of
the program you are debugging.  This includes the effects of statements in
your program that you trace.  The size of this history is limited only by
available memory.  Undo and Redo allow you to step backward and forward
through this history.  This allows you to reverse the effects of tracing
over simple statements in your program.  You can also reverse any accidental
interactions that affect your program's state.  See The Undo Menu.