Orna Grumberg Martin Lange Martin Leucker Sharon Shoham <i>Don't Know</i> in the µ-Calculus. 233-249 http://dx.doi.org/10.1007/978-3-540-30579-8_16 2005 conf/vmcai/2005 VMCAI db/conf/vmcai/vmcai2005.html#GrumbergLLS05