There was a composition student who could not write a symphony. He had to hand in his homework the next day, so he went to ask his professor how to write a symphony in one day. . The professor probably drank too much at the time, so he said: "There is only one way, that is, you take a symphony, copy it backwards, treat the last note as the first, and copy it backwards to become a symphony. Symphony *** IV Just turn it upside down." He said, "That's great, thank you, Professor." When I got home, I only had the professor's symphony at hand, so I copied it backwards. Handed in the homework. As a result, when the piece was played, it turned out to be Beethoven's "Fifth Symphony". It turned out that the professor's work was copied from Beethoven's work backwards, and the student copied it back again.