Noboru Matsuda Toshio Okamoto Diagrammatic Reasoning for Geometry ITS to Teach Auxiliary Line Construction Problems. 244-253 http://link.springer.de/link/service/series/0558/bibs/1452/14520244.htm 1998 conf/its/1998 Intelligent Tutoring Systems db/conf/its/its1998.html#MatsudaO98