� 1999 Massachusetts Institute of Technology
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Library of Congress Cataloging-in-Publication Data

The University of Chicago Press, Chicago 60637
The University of Chicago Press, Ltd., London
� 1999 by the National Bureau of Economic Research
All rights reserved. Published 1999
08070605040302010099 12345
ISBN: 0-226-24103-3 (cloth)
ISBN: 0-226-24104-1 (paper)