Analysis-Separate US talks with Canada, Mexico test North America's trilateral trade pact
By Promit Mukherjee, Emily Green and David Lawder OTTAWA/MEXICO CITY, July 22 (Reuters) - Separate U.S. negotiating tracks for Canada and Mexico are testing a trilateral free trade zone that has defined North America's economy for 32 years, with the U.S. potentially forcing concessions on Canada 
