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.

The vote, expected in the early hours of Friday, will test the strength of a rebellion by anti-austerity Syriza lawmakers, which could raise pressure on Prime Minister Alexis Tsipras to call snap elections as early as September.

Answer: chairperson