CANCUN, Mexico - President Bush said Friday the United States believes it is important to enforce laws protecting borders and told the leaders of Mexico and Canada that was crucial to keeping prosperity alive. ADVERTISEMENT He also reiterated strong support for a "guest worker" program that would allow undocumented immigrants already in the United States to remain in the country to fill low-paying jobs that Americans won't take. Bush declined to say whether he would veto legislation that did not contain such a provision. "I want a comprehensive bill," Bush said at a joint news conference with Mexican President Vicente...