Antonija Mitrovic Pramuditha Suraweera Brent Martin Konstantin Zakharov Nancy Milik Jay Holland Authoring Constraint-Based Tutors in ASPIRE. 41-50 2006 conf/its/2006 Intelligent Tutoring Systems http://dx.doi.org/10.1007/11774303_5 db/conf/its/its2006.html#MitrovicSMZMH06