1. Van Wijngaarden, A., Mailloux, B. J., Peck, J. E. L., and Koster, C. H. A., Report on the Algorithmic Language ALGOL 68. MR 101 Mathematisch Centrum, Amsterdam, October 1969. Also Numerische Mathematik 14 (1969) 79–218.
2. Cheatham, T. E., Jr., Fischer, A., and Jorrand, P., On the Basis for ELF-An Extensible Language Facility. Proc. AFIPS 1968 Fall Joint Comput. Conf., Vol. 33 Pt. 2, MDI Publications, Wayne, Pa., pp. 937–948.
3. Reynolds, J. C., A Set-theoretic Approach to the Concept of Type. Working paper, NATO Conf. on Techniques in Software Engineering, Rome, October 1969.
4. Morris, J. H., "Protection in Programming Languages," Comm. ACM, 16 (1), January 1973.
5. Morris, J. H., Types are not Sets. Proc. ACM Symposium on Principle of Programming Languages, Boston 1973, pp. 120–124.