Ollabelle is an ancient-to-modern roots-fusion act, currently warming up for its role as backing band at the upcoming Summerstage tribute to Simon & Garfunkel. Tonight is billed as a rehearsal for the fancy gala; you sure can't go wrong with S&G's songs, and Ollabelle is a formidable live proposition, shifting effortlessly from deep country-rock to hymnlike chanting and hypnotic twang.