TOKYO/SINGAPORE A weak eco**my, deflation, massive public debt, negative interest rates and an ageing citizenry don't seem like good reasons for a country?s currency to surge, but that?s exactly what happened to Japan?s yen after Britain's vote to leave the European Union. The safe-haven phe**me**n is a huge headache for the Japanese government with few easy options to curb a rise that badly hurts the nation?s exporters and could tip the eco**my back into recession.