Samuli Heilala Brigitte Pientka Bidirectional Decision Procedures for the Intuitionistic Propositional Modal Logic IS4. 116-131 2007 conf/cade/2007 CADE http://dx.doi.org/10.1007/978-3-540-73595-3_9 db/conf/cade/cade2007.html#HeilalaP07