Solving the $100 modal logic challenge.

Florian Rabe Petr Pudlák Geoff Sutcliffe Weina Shen Solving the $100 modal logic challenge. 113-130 2009 7 J. Applied Logic 1 http://dx.doi.org/10.1016/j.jal.2007.07.007 db/journals/japll/japll7.html#RabePSS09