Help! When I enter into trade agreements, I always hear from my opponent when the trade agreement expires (after 20 turns) -- unless the trade involves a critical resource I am trading to them. For example, I sent iron to the French and the Iroqouis. This trade never comes up for review and I don't know how to cancel/renogotiate. They gouge me every 20 turns for stuff I'm getting from them (initiated in a different trade), but I can't up the ante for my iron (I may have traded iron originally for technology and lump sum gold, not a turn-by-turn deal -- is that the problem?) I know I could get a leg up on my current match if I could get my iron back and negotiate new deals! 
