Madonna caused a minimal stir by sniping at Lady Gaga, referencing her during a concert and adding, “Imitation is the sincerest form of flattery.” At Chicago’s United Center, the first of two concerts there this week, Madonna again slipped the chorus of Gaga’s “Born This Way” into the bridge of her own “Express Yourself” — i t’s a seamless match, for sure — but let it go without comment. Well, almost. She shouted a bit from “She’s Not Me” at the end. It seems like pretty catty paranoia from the indisputable queen of pop, as if the Material Girl — a 1 percenter if ever there were — has adopted the Republicans’ new slogan (“We built it!”) and its false sense of rugged individualism. Madonna broke ground for women in pop during the ’80s and easily can justify her worldwide love, but her success is a pastiche quilt, a smart synthesis of the best of the best. Wednesday’s show only lengthened the long list of film and mu