13. The system of claim 12 , wherein: a plurality of documents in the set of documents each have a resource locator; the instructions for grouping the set of documents into a plurality of clusters include instructions for: ordering the set of documents in accordance with the resource locators; and selecting a respective plurality of consecutive documents from the ordering for inclusion in the respective cluster.
14. The system of claim 13 , wherein: the resource locator is a URL; and the respective cluster includes documents from a particular sub-domain within a same domain.