To Potsdam in 1747 went Johann Sebastian Bach, by royal invitation, and was joyfully received by "the most distinguished flute player in Europe," King Frederick the Great. Himself an amateur versifier and composer, Frederick asked his famed visitor to improvise on a royal theme. Bach forthwith obliged, extemporized a three-voice fugue. Frederick, delighted but still royal, thereupon set a more difficult task: development of his original theme into a six-part fugue. That was a stumper, even for the 62-year-old master; to make things worse, Frederick wished the assignment worked out in his presence.
Bach demurred, gracefully. But the King of Prussia's poser...