Larry Wos Dolph Ulrich Branden Fitelson Vanquishing the XCB Question: The Methodology Discovery of the Last Shortest Single Axiom for the Equivalential Calculus http://arxiv.org/abs/cs.LO/0211014 2002 CoRR cs.LO/0211014 db/journals/corr/corr0211.html#cs-LO-0211014 informal publication