Luca Pulina Armando Tacchella Treewidth: A Useful Marker of Empirical Hardness in Quantified Boolean Logic Encodings. 528-542 2008 LPAR http://dx.doi.org/10.1007/978-3-540-89439-1_37 conf/lpar/2008 db/conf/lpar/lpar2008.html#PulinaT08