TOKYO -- The Bank of Japan is growing into its role as a key source of support for the country's stock market, as it has stepped up purchases of exchange-traded funds to bring its equities portfolio to an estimated 7 trillion yen ($63.6 billion) or so. The central bank bought 123.6 billion yen worth of ETFs in August, the largest monthly tally so far this year. At one point, it snapped up ETFs in