NEW YORK - The battle with the United States over China's exchange rate continues. When the Great Recession began, many worried that protectionism would rear its ugly head. True, G-20 leaders promised that they had learned the lessons of the Great Depression. But 17 of the G-20's members introduced protectionist measures just months after the first summit in November 2008. The "Buy America" provision in the United States' stimulus bill got the most attention. Still, protectionism was contained, partly due to the World Trade Organization.