Mark Morris essays a Sylvia, and it’s a serious ballet, writes Joan Acocella. “In its classicism, “Sylvia” could start a fashion. The big ballet companies are always crying out for three-act story ballets, because that’s what the public wants. And what they end up with, usually, is either some same-old revision of “Giselle” or “Swan Lake” (the San Francisco troupe has these) or the opposite, a new-style, lurid, hauling-the-girls-by-the-crotch melodrama (San Francisco also has one of these, Lar Lubovitch’s “Othello”). “Sylvia” could point a new way: both purely classical—a symbol, not a soap opera—and also serious.”