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.

Greek Prime Minister Alexis Tsipras on Wednesday had to face down a major mutiny in his radical left Syriza party, and violent protests in the streets of Athens, to win parliamentary approval for the draconian reforms demanded in the bailout package.

Answer: chairperson