The Rochester Oratorio Society is rehearsing Carmina Burana, the secular cantata by Carl Off, composed in the 1930’s. It's based on a thirteenth century manuscript discovered in a Bavarian monastery. Beloved by singers and derided by critics for its lack of polyphony, Carmina celebrates spring, sex, love, and drinking, all while bemoaning the vagaries of fate. It’s fun stuff. In the upcoming May performance, the conductor has decided to use the “Coro Piccolo,” that is, to have a small chorus sing some of the sections instead of the full choir singing everything. This doesn’t please those left out, and during last night’s rehearsal, a few confessed to feeling resentful. “I KNOW that part,” said one soprano chosen to sit out during the small chorus sections.