The emerging strength of the far left and far right is creating awkward choices for mainstream parties after the first round of French mayoral elections. In many big towns and cities, Socialists and centre-right Republicans are tempted to make electoral pacts on their outside flanks in order to beat the opposition in next Sunday's run-off. But alliances with the National Rally (RN) on the far right or France Unbowed (LFI) on the far left carry big risks as well as opportunities. Take Marseille. There, after round one, the incumbent Socialist Mayor, Benoît Payan, is only a whisker ahead of RN candidate Franck Allisio. But two other candidates have also qualified for round two, with lower scores: the Republicans' Martine Vassal and the LFI's Sébastien Delogu. So should the Socialists enter a pact with LFI in order to save Payan? And should Vassal throw her lot in with RN in order to keep out the left?…