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Gun owners and advocates are fond of saying, “Guns don’t kill people, people kill people.” This might be a more useful aphorism: Smart-guns don’t kill the wrong people. Technology exists, or could exist, that would make guns safer. The idea of a safe gun might seem to be the ultimate oxymoron: guns are designed to kill. But something missing from the gun-control debate that has followed the killing of 20 children and six adults at an elementary school in Newtown, Conn., is the role of technology in preventing or at least limiting gun deaths.