Comment 4 for bug 1525048

I misspoke. What I meant to say was, if you don't want to freeze Subject and Date, for example, you must remove them from frozen_sign. Since frozen_sign is a set, you must set the entire set to a new set (you can't del frozen_set['Subject'] for example). A user needs to know to add From to frozen_set or to should_sign, and if nothing else, this isn't clear from the available documentation.