There is a terrible inevitability about the bombing in Charsadda, Pakistan, on Friday morning. Little about it is different from previous bombings. There is the same vicious tactic of two devices, the latter apparently arriving on a donkey cart, with one designed to kill helpers. There is a familiar target: hapless recruits to the underpaid, under-equipped paramilitary frontier corps. There is a familiar culprit: the Pakistani Taliban, who claimed the attack.