Antonija Mitrovic Amali Weerasinghe Revisiting Ill-Definedness and the Consequences for ITSs. 375-382 2009 AIED http://dx.doi.org/10.3233/978-1-60750-028-5-375 conf/aied/2009 db/conf/aied/aied2009.html#MitrovicW09