1     The author gratefully acknowledges the help of the Outlooks team at the Ministry of Transportation and Communications, Province of Ontario (this model was developed through intimate interaction with them during summer 1981 while completing Project 80130), Leonhard Euler (1707-1783) who invented this technique of representing relationships to teach logic in his Letters to a German Princess, and of Fred Knelman (1919-??) of Concordia University, who suggested this particular use of the Euler diagram.

2     Miller, George A. The magical number seven, plus or minus two: Some limits on our capacity for processing information. Psychological Review. 1956, 63, pp. 81-97.

3     Simon, Herbert A. Sciences of the Artificial. Cambridge, Mass: MIT Press, 1969.