Scary undo

[Lennart Borgman (gmail)]

I would like to undo a change made by a function that I call with 
funcall. I do not know exactly what the function does, but hopefully it 
will only indent the current line. However the new indentation might be 
wrong.

The scenario is something like this

   (funcall the-indentation-function)
   (when (= 0 (current-indentation))
      ;; undo what `the-indentation-function' did,
      ;; but how?
      )

I am a bit scared of writing this undo part so if someone could help me 
I would be glad. I do not think I can use `undo-boundary' here.