cogens—this term is applied in modern international law to indicate peremptory norms that cannot be derogated from). These are regulations that cannot be changed or excluded by party agreement. Those regulations that can be changed are called today ius dispositivum, and they are not used when party shares something and are in contrary. Public law The Roman Republic's constitution or mos maiorum ("custom of the ancestors") was an unwritten set of guidelines and principles passed down mainly through precedent. Concepts that originated in the Roman constitution live on in constitutions to this day. Examples include checks and balances, the separation of powers, vetoes, filibusters, quorum requirements, term limits, impeachments, the powers of the purse, and regularly scheduled elections. Even some lesser used modern constitutional concepts, such as the block voting found in the electoral college of the United States, originate from ideas found in the Roman constitution. The constitution of the Roman Republic was not formal or even official. Its constitution was largely unwritten, and was constantly evolving throughout the life of the Republic. Throughout the 1st century BC, the power and legitimacy of the Roman constitution was progressively eroding. Even Roman constitutionalists, such as the senator Cicero, lost a willingness to remain faithful to it towards the end of the Republic. When the Roman Republic ultimately fell in the years following the Battle of Actium and Mark Antony's suicide, what was left of the Roman constitution died along with the Republic. The first Roman emperor, Augustus, attempted to manufacture the appearance of a constitution that still governed the Empire, by utilising that constitution's institutions to lend legitimacy to the Principate, e.g., reusing prior grants of greater imperium to substantiate Augustus' greater imperium over the imperial provinces and the prorogation of different magistracies to justify Augustus' receipt of tribunician power. The belief in a surviving constitution lasted well into the life of the Roman Empire. Private law Stipulatio was the basic form of contract in Roman law. It was made in the format of question and answer. The precise nature of the contract was disputed, as can be seen below. Rei vindicatio is a legal action by which the plaintiff demands that the defendant return a thing that belongs to the plaintiff. It may only be used when plaintiff owns the thing, and the defendant is somehow impeding the plaintiff's possession of the thing. The plaintiff could also institute an