Alexandru Baltag Sonja Smets Learning by Questions and Answers: From Belief-Revision Cycles to Doxastic Fixed Points. 124-139 2009 WoLLIC http://dx.doi.org/10.1007/978-3-642-02261-6_11 conf/wollic/2009 db/conf/wollic/wollic2009.html#BaltagS09