Issues in relation to which referendums are optional at the national level

Slovakia
CONSTITUTIONOF THE SLOVAK REPUBLIC
(Adopted 1992; lastamended 2006)
http://www.nrsr.sk/web/default.aspx?SectionId=124
Chapter5: Legislative Power
Part2: The Referendum
Article93
(1) Areferendum is used to confirm a constitutional law on entering into a unionwith other states, or on withdrawing from that union.
(2) Areferendum can be used to decide also on other important issues of publicinterest.
(3) Basicrights and freedoms, taxes, levies and the state budget may not be the subjectof a referendum.
Article95