Oliver Kullmann Inês Lynce João Marques-Silva Categorisation of Clauses in Conjunctive Normal Forms: Minimally Unsatisfiable Sub-clause-sets and the Lean Kernel. 22-35 2006 conf/sat/2006 SAT http://dx.doi.org/10.1007/11814948_4 db/conf/sat/sat2006.html#KullmannLM06