NIGERIA'S two main opposition leaders asked the Supreme Court to quash last month's tribunal ruling upholding President Bola Tinubu's February election victory, in a last bid to reverse the results of a vote widely accepted by the international community. No legal challenge to the outcome of a presidential election has succeeded in Nigeria, which returned to democracy in 1999 after three decades of almost uninterrupted military rule and has a history of electoral irregularities. Elections in the past had been characterised by ballot box snatching, accusations of allowing unregistered voters to cast ballots, and completely made-up results. Atiku Abubakar of the People's…