Given the input sentence, please extract the subject and object containing a certain relation in the sentence according to the following relation types, in the format of "relation1: word1, word2; relation2: word3, word4". Relations include: 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.

Singer also took aim at what he called the "cult of central banking" in which investors turned to US Federal Reserve chair Janet Yellen and European Central Bank president Mario Draghi to solve the problems of the global financial system.

Answer: position_held: Janet Yellen, president