For his 2009 revamp of San Francisco Ballet’s “Swan Lake,” artistic director Helgi Tomasson added a Prologue. The idea was to make this Odette’s story, but if you ask me it’s still all about Siegfried. We may see Odette first, witnessing her capture by Von Rothbart and transformation into an animal, but Siegfried is still the character who has, as the narratologists might say, “agency.” He’s the one who makes the wrong choice and must pay penance; his is the narrative arc.