Poland was the first country in Europe to adopt a written constitution (of 3rd May 1791). Subsequent constitutions were adopted after the restoration of independence which took place in 1918. These were the March Constitution of 1921 and April Constitution of 1935. Both acts varied considerably. After World War II Poland lost its capacity to shape its system of government in an independent way. This was manifested in the adoption, in 1952, of the Constitution of Polish People's Republic, modelled on the 1936 Soviet constitution (also known as the Stalin Constitution). Since 1989, fundamental changes in the system of government have been introduced in Poland. The current Constitution of the Republic of Poland was adopted by the National Assembly on 2 April 1997. A nationwide referendum on its approval was held on 25 May and its came into force on 17 October 1997. Poland's Constitution is composed of 13 chapters preceded by a preamble. The most important principles contained in the Constitution are the principles of: a republican form of government, the common good, subsidiarity, democratic state governed by the rule of law, sovereignty of the nation, separation of and balance between powers, political pluralism, recognition of inherent dignity of the person as a source of his/her freedoms and rights, and freedom of economic activity. Some of these principles, rooted in classical constitutional law, have been considerably changed in the process of European integration. The principle of national sovereignty is a good example of it. In the 10-year period of being in force, the Constitution proved itself as an act difficult to be amended. According to legal science, it is classified as a 'fixed' constitution, which means the procedures for its amendment contain various types of thresholds (e.g. the requirements concerning majority of votes) that are difficult to meet. Effectiveness of those barriers have been confirmed by the fact that, until now, the Constitution was amended only once (on 8 October 2006) to include the provisions concerning the European arrest warrant (EAW). As of 17 May 2007, there were five bills to amend Poland's Constitution submitted to the Sejm. One of those bills related to assistant judges adjudicating in district courts, another two bills related to formal immunity of the Deputy to the Sejm, and next two proposed implementing a prohibition on the election to the Sejm of persons convicted for certain intentional offences. .