* @param stats translation / solving stats
* @return the result of solving an unsat formula.
*/
private static Solution unsat(Translation translation, Statistics stats) {
final SATSolver cnf = translation.cnf();
final TranslationLog log = translation.log();
if (cnf instanceof SATProver && log != null) {
return Solution.unsatisfiable(stats, new ResolutionBasedProof((SATProver) cnf, log));
} else { // can free memory
final Solution sol = Solution.unsatisfiable(stats, null);
cnf.free();