State Street Global Advisors -- the provider of the mammoth SPDR ETF family -- added two bond funds this week: SPDR BofA Merrill Lynch Crossover Corporate Bond ETF (XOVR) and the SPDR BofA Merrill Lynch Emerging Markets Corporate Bond ETF (EMCD) . XOVR tracks the BofA Merrill Lynch U.S. Diversified Crossover Corporate Index, which is a basket of dollar-denominated BBB and BB corporate bonds. Crossover refers to credit quality. Crossover corporate