What is the relationship between Syriza and Alexis Tsipras in the context of the input sentence. Choose an answer from: product/material produced; manufacturer; distributed by; industry; position held; original broadcaster; owned by; founded by; distribution format; headquarters location; stock exchange; currency; parent organization; chief executive officer; director/manager; owner of; operator; member of; employer; chairperson; platform; subsidiary; legal form; publisher; developer; brand; business division; location of formation; creator.

Having reluctantly agreed to stringent terms for a new bailout deal, Greek Prime Minister Alexis Tsipras must face down a rebellion in his anti-austerity Syriza party to push sweeping pro-market reforms and spending cuts through parliament.

Answer: chairperson