Filtering of search results by access constraints.
This was part of the plan in https://conferences.tdl.org/or/OR2011/OR2011main/paper/view/416/127 for GSearch 2.4.
Based on work presented at OR2008 : http://pubs.or08.ecs.soton.ac.uk/104/
Problem: Search results contain hits that the user does not have the access rights to see
Solution: Extend access rights to search results by filtering