László Egri Benoit Larose Pascal Tesson Directed st-Connectivity Is Not Expressible in Symmetric Datalog. 172-183 2008 ICALP (2) http://dx.doi.org/10.1007/978-3-540-70583-3_15 conf/icalp/2008-2 db/conf/icalp/icalp2008-2.html#EgriLT08