Dear Economist,
  There is a legend about the last king of the Romans, Tarquin. An old witch  came to Tarquin, and offered to sell him nine books of prophecy at an exorbitant  price. Tarquin laughed at the offer. The witch burned three of the books, and  then offered to sell him the remaining six for the original price. Tarquin  refused again.
  The witch burned three more books and offered to sell Tarquin the three  books that were left for the original price that she had demanded for nine. This  time Tarquin was scared that he might be losing something precious, and bought  the remaining three books for the price that the witch asked. What sort of  demand curve is that?
  Chris McMahon, by e-mail
  克里斯o麦克马洪(Chris McMahon)通过电子邮件发送
  Dear Mr McMahon,
  Forget the demand curve; this is a two-player negotiation over the division  of economic surplus. Tarquin was always willing to pay a high price but hoped to  get a bargain. The sibyl ("witch" is such an uncouth label) responded with a  supply constriction designed to drive up the price.
  Tarquin might have thought that the sibyl had just one rival buyer, and if  each buyer wanted only one trilogy, that would be a supply glut. Once there was  only one trilogy available for two buyers, Tarquin knew he was in a serious  auction and made a pre-emptive offer.
  Another possibility is that the sibyl was dealing with the so- called  durable monopoly problem. Tarquin knew that the sibyl might sell him an  expensive trilogy, and then come back later with a cut- price offer to buy a  second or third. By destroying two trilogies, the sibyl enabled herself to make  a credible, take-it-or-leave-it offer. Forward-thinking stuff, but then, she was  flogging prophecies.