20. The system of claim 19 , wherein the document ranking engine is configured to rank more highly search results having a subject-version tag matching the subject-version of the search query than search results which lack a subject-version tag matching the subject-version of the search query.
19. The system of claim 17 , wherein the document index is configured to store more than one subject-version tag for a search result if the search result is associated with more than one subject-version.