Organizing human knowledge into related areas is nearly as old as human knowledge itself, as is evident in writings from many ancient civilizations. In modern times, the task of organizing knowledge into systematic structures is studied by ontologists and library scientists, resulting in such well-known structures as the Dewey decimal system, the Library of Congress catalog, the AMS Mathematics Subject Classification, and the U.S. Patent Office subject classification [11, 68]. Subject-based organization routinely permeates our personal lives as we organize books, CDs, videos, and email.

The evolution of the Web has followed this familiar history. Around the same time as ad hoc keyword search engines like AltaVista ...

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