Note for programmers: "undoable" is something that can't be done, not something that you can undo.
and what's the correct answer? - Pavel Kaplin
"The prefix un means not, reverse action, deprive of, release from" (http://www.ecenglish.com/learnen...). So it's really something you can undo. What can't be done could be called non-doable, im- or dis- could perhaps be used but nobody says so. - Peter Gromov
what's wrong with good old "impossible", anyway? - Evelynne