How do I protect the user's system variables better?

My program uses a simple error catch as the only exit point. The problem is the user can undo the setvar calls that reset the system variables and leave the variables in a undesirable state. "Undo" "C" "NONE" will make all previous actions undoable. How do we make it impossible to undo the setvar's while not blowing away the entire undo history?

I use this structure:

(defun c:mycommand ( / *error* )

   (defun *error* ( m )
       [color=green]< Reset System Variables >[/color]
       (LM:endundo (LM:acdoc))

       < ... >
   )

   (LM:startundo (LM:acdoc))
   [color=green]< Set System Variables >[/color]

   < ... >

   [color=green]< Reset System Variables >[/color]
   (LM:endundo (LM:acdoc))
)

With the following library functions:

[color=GREEN];; Start Undo  -  Lee Mac[/color]
[color=GREEN];; Opens an Undo Group.[/color]

([color=BLUE]defun[/color] LM:startundo ( doc )
   (LM:endundo doc)
   ([color=BLUE]vla-startundomark[/color] doc)
)

[color=GREEN];; End Undo  -  Lee Mac[/color]
[color=GREEN];; Closes an Undo Group.[/color]

([color=BLUE]defun[/color] LM:endundo ( doc )
   ([color=BLUE]while[/color] ([color=BLUE]=[/color] 8 ([color=BLUE]logand[/color] 8 ([color=BLUE]getvar[/color] 'undoctl)))
       ([color=BLUE]vla-endundomark[/color] doc)
   )
)

[color=GREEN];; Active Document  -  Lee Mac[/color]
[color=GREEN];; Returns the VLA Active Document Object[/color]

([color=BLUE]defun[/color] LM:acdoc [color=BLUE]nil[/color]
   ([color=BLUE]eval[/color] ([color=BLUE]list[/color] '[color=BLUE]defun[/color] 'LM:acdoc '[color=BLUE]nil[/color] ([color=BLUE]vla-get-activedocument[/color] ([color=BLUE]vlax-get-acad-object[/color]))))
   (LM:acdoc)
)

When considering the order of operations & Undo Group markers within the program & error handler, be aware that when an Undo call is issued, AutoCAD will undo all operations following the end of an Undo Group before Undo'ing the Group.

Thanks Lee Mac. I didn't mention I needed it to step back through a loop, but your post gave me the pattern to make it work. Now, I just need to learn the fancy VLA stuff so I quiet it down....(command "_.undo...) is noisy!

You're welcome!

(command "_.undo...) is noisy!

Temporarily disable CMDECHO