Musicals, like films, sometimes have sequels, though the history of them is littered with more failures than successes, as witness the heavily abbreviated runs of Bring Back Birdie (the sequel to Bye Bye Birdie), The Best Little Whorehouse Goes Public (The Best Little Whorehouse in Texas) and Annie 2: Miss Hannigan's Revenge (which closed out-of-town before reaching Broadway).But Musik is something entirely different: it's a very ripe, clever and frequently hilarious cabaret spin-off from a 2001...