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TERRORISM is most commonly associated with the bomb and the bullet, but ever since an incident on the Tokyo subway system in 1995, the security services have also had to worry about poison gas. That attack, which used a nerve-gas called sarin, killed 12 people and severely injured another 50. Sarin is to be feared because it is invisible, odourless and 500 times more deadly than cyanide. But other gases (not least cyanide itself) could be used instead.