Is there an overarching rule for terms that need undoing, specifically for actions taken that need to be revoked or reversed? Which prefix, 'de-' or 'un-', is preferred in such cases? Is there a distinction created by the underlying terms? Is it purely a usage issue where the one most used is the one to use ________. Do 'un-' and 'de-' actually have different meanings that could help determine which is most appropriate?