Celebrating Sir Frederick Ashton’s 100th birthday, the Royal Ballet is restaging Sylvie. But the work hasn’t been done since 1965 and all but two of those involved with the production are dead. And there are few records of what the piece looked like. “Some might wonder how the Royal Ballet found itself in this situation in the first place, by failing to record the work of its founder choreographer. But notation was in its infancy at the time, and Sir Frederick himself was disdainful of posterity.”