In Beethoven's Choral Fantasy, about 6 or 7 minutes in, different sections of the orchestra play the theme in different variations. One such variation comes in the form of a solo flute playing over the piano. The piano is playing a line from Mozart's "The Magic Flute," an opera about the signifigance and power of music, the same subject as the Choral Fantasy. The flute is not playing Mozart, but it is interesting to note that Beethoven chose to have a flute solo over this paino line, obviously a reference to a composer well-respected by the great Ludwig van.