AN explosion on the grounds of the Philippine presidential palace today was probably caused by chemicals in a trash bin set off by a lit cigarette, not a bomb, the head of security said. "Nobody was injured," press undersecretary Isabel de Leon said, adding President Gloria Macapagal Arroyo was inside the palace at the time. The army tightened security yesterday after finding a bomb in its top military academy, one of the alleged targets of a plot to overthrow and possibly kill Ms Arroyo, who survived an impeachment attempt last year over allegations of vote-rigging and graft. Witnesses said the...