(Bloomberg) -- Andy Maack's vision of a fair currency market doesn't include last look, a controversial practice that allows dealers to back out of losing trades.

Maack's view matters because he's head of foreign-exchange trading at the world's largest mutual-fund company, Vanguard, which manages about $4 trillion. While the use of last look has diminished in recent months in the $5.1-trillion-a-day currency market, it still persists, he said.

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