Masahiko Sato Yukiyoshi Kameyama Izumi Takeuti CAL: A Computer Assisted Learning System for Computation and Logic. 509-524 http://link.springer.de/link/service/series/0558/bibs/2178/21780509.htm 2001 conf/eurocast/2001 EUROCAST db/conf/eurocast/eurocast2001.html#SatoKT01