Stephen Sondheim calls "Sweeney Todd" a musical thriller," and it certainly is. So are "Don Giovanni" and "Otello" and "Turandot" and "Carmen." Any time a case needs to be made for sheer musical and theatrical genius in American opera, "Sweeney Todd" can be entered in the winning column.

Labels don't matter, but Sondheim's "Todd" can trace its lineage straight back to some of the greatest names in operatic history. Like Wagner and Menotti, Sondheim wrote his own words for his music. And those words have some of the biting wit of DaPonte, who wrote well for Mozart, and Boito's great style for Verdi. There are kinds of variety in dramatic situations that Verdi so often begged his earlier librettists to give him. And there are gorgeous examples of what Verdi had in mind when he said, "At that point I would write a little music."

Such music in Sondheim's score! His voices are used in solos that range from tender lyricism and wild humor to searing scorn and scorching bitterness. He uses them alone and in duo, trio, quartet and quintet, and fashions his chorus into a mobile ensemble that varies in sound from a touching chamber group to a roaring London mob.

His orchestra, too, has, at times, the blackness of "Rigoletto," and the pathos often heard in Menotti. As with every great opera, it establishes the atmosphere of succeeding scenes, subtly underlining all that happens on the stage. It is no surprise that, notably toward the end, there are echoes of Bernstein. In other words, Sondheim knows great sources for opera. But he has written pure Sondheim and done it brilliantly.

It is no wonder that his superb cast makes the most of its opportunities. That is often the case when singing actors are given the kinds of challenges with which "Sweeney Todd" is filled. It is fortunate the singers do not carry the burdens of routined opera singers, few of whom could equal the great things seen and heard in "Todd." The title role is so closely related to Rigoletto that Sondheim may hear it taken up by operatic baritones before long.