Can anyone please explain to me the following: I'm listening for the first time to what is a most delightful opera - Die Fledermaus by Strauss - I know, I know, very late to the party but I recognize so many of the tunes. But in the middle of the opera there's something called the "gala scene" where guest singers (on this recording by von Karajan from 1960, which is wonderful( suddenly start singing other music, including I Could Have Danced All Night, Porgy and Bess, and Annie Get Your Gun. What is THAT about? Is this standard?