22. The computer readable storage medium of claim 14 , further including an equivalence rule validation module for: selecting a set of validation URLs for the equivalence rule; checking if the equivalence rule correctly predicts a document content type for each of the validation URLs; and repeating said generating and grouping until said checking determines that the equivalence rule correctly predicts a document content type for each of the validation URLs.
23. The computer readable storage medium of claim 22 , wherein a URL more different than another one from the selected validation URLs is given a higher priority in the selection of validation URLs.