1. Nature, 250, 533–534 (1974).
2. Higman, B. A., A comparative study of programming languages (Elsevier, Amsterdam, 1967).
3. Moore, J., and Newell, A., Knowledge and Cognition (edit. by Gregg, L.) (Lawrence Eribaum Associates, Potomac, 1973).
4. Kelley, M. D., Machine Intelligence, 6, 397–409 (Edinburgh University Press, Edinburgh, 1971).
5. Raphael, B., in Proc. Fall Joint Computer Conference (Association for Computing Machinery, New York, 1964).