1. Joint Research Center for Logic, Department of Philosophy, Tsinghua University, Beijing, 100084, China, ILLC, University of Amsterdam, Amsterdam, P.O. Box 94242, The Netherlands
2. Department of Philosophy, Stanford University, California 94305, USA
3. Joint Research Center for Logic, Department of Philosophy, Tsinghua University, Beijing, 100084, China
4. Department of Computer Science and Technology, Tsinghua University, Beijing, 100084, China