THREE opposition candidates in Senegal's presidential election filed legal challenges to the president's abrupt postponement of the election, as riot police fired tear gas to break up a protest outside parliament. While protesters shouted slogans on the streets, lawmakers inside the National Assembly building debated into the evening a bill that would reschedule the February 25 vote to August 25 and extend President Macky Sall's mandate until his successor is installed. Sall's unprecedented announcement on Saturday pitched the West African nation into uncharted constitutional waters that threaten to tarnish its reputation as a bastion of democratic stability in a region swept by…