Christoph Behle Andreas Krebs Mark Mercer Linear Circuits, Two-Variable Logic and Weakly Blocked Monoids. 147-158 2007 conf/mfcs/2007 MFCS http://dx.doi.org/10.1007/978-3-540-74456-6_15 db/conf/mfcs/mfcs2007.html#BehleKM07