E. Anderson, F. Belz, and E. Blum. Mathematical Studies of Information Processing, volume 75 of Lecture Notes in Computer Science, page 384-424. Springer, (1978)
N. Anderson, and C. Lin. ITiCSE '09: Proceedings of the 14th annual ACM SIGCSE conference on Innovation and technology in computer science education, page 243--247. New York, NY, USA, ACM, (2009)