Paulus, On the Manner of Drawing up Formulas. Exceptions are pleaded either because the party did what he should have done; or because he did what he ought not to have done; or because he did not do what he should have done. An exception on the ground of property sold and delivered, or on that of res judicata, is granted for the reason that something has been done which ought to have been done. An exception on the ground of fraud is granted, because something has been done which ought not to have been done. An exception on the ground that prætorian possession of property which has been given has not been permitted, is granted because something was not done which should be done.