Since its Broadway premiere in 1979, "Sweeney Todd" has lent itself to many different treatments, from chamber musical to full-fledged performance at some of the world's leading opera houses. And its rich, complex score begs the question: Is "Sweeney" a musical or an opera in disguise? Bryn Terfel, who plays "Sweeney," tells us what he's learned from his own encounter with this daunting work.