"In 1993, oil and gas corporations based in Canada, many of them foreign-owned, lobbied for a proportionality clause to be included in NAFTA, which was then being negotiated. Under proportionality, Canada can cut exports to the U.S. to deal with shortages, only if we cut the same proportion of supplies to Canadians. This would not help at all."

http://www.theglobeandmail.com/servlet/story/RTGAM.20050217.webcolaxer16/BNStory/National/

Professor, if I'm not mistaken that also applies to Mexico? Good deal for Us ;)