Anyone attending the premiere of Berlioz's rarely seen opera "Benvenuto Cellini", directed by Monty Python alumnus Terry Gilliam at the English National Opera, could be forgiven for thinking it was a preview for the Python reunion show in July. Gilliam, who seems to hate a void, filled the stage of London's enormous Coliseum theater on Thursday with jugglers, trapeze artists, stiltwalkers and tumblers for one of the 19th-century French composer's most troubled works. Huge papier-mache-style masks of a devil and a skull were paraded down the aisles within minutes of the curtain going up and they remained suspended from boxes on either side of the stage for the duration, emphasizing the carnival tone. true "Benvenuto Cellini", Berlioz's first opera that was based on the life of the renowned 16th-century Italian sculptor, was a flop when it had its premiere in Paris in 1838 and has been revived only sporadically since. In Gilliam's re-