For opera fans, summer is the time for the Merola Opera Program, one of the top young artist residencies in the country. The singers getting a polishing at Merola are literally one step away from the major stages of the world. Some will be stars. But can you guess which ones? The Schwabacher Summer Concert is your first chance to tell, as well as to hear extended scenes from some not-the-usual -warhorse operas: Donizetti’s Anna Bolena, Stravinsky’s Rake’s Progress, Boito’s Mefistofele, and Bizet’s La jolie fille de Perth. (If you can hum an aria from that last one, maybe you can convince the box office to give you a free ticket in.)