This was also the source of the tribunes' power, known as "ius intercessionis," or "intercessio," by which any tribune could intercede on behalf of a Roman citizen to prohibit the act of a magistrate or other official. Citizens could appeal the decisions of the magistrates to the tribunes, who would then be obliged to determine the legality of the action before a magistrate could proceed. This power also allowed the tribunes to forbid, or "veto" any act of the senate or another assembly. Only a dictator was exempt from these powers.