1. Batey, M., Ed. Working Draft of ECMA/ANSI PL/I Standard Tenth Rev., ANSI, New York, (Sept. 1973).
2. Birkhoff, G., and Lipson, J.D. Heterogeneous algebras. J. Combinatorial Theory 8(1970), 115–133.
3. Dahl, O.-J., Nygaard, K., and Myhrhaug, B. The SIMULA 67 Common Base Language. Norwegian Comptng. Centre, Oslo, 1968.
4. Dijkstra, E.W. Notes on structured programming. InStructured Programming, Academic Press, New York, 1972.
5. Floyd, R.W. Assigning Meaning to Programs., Proc Symp. in Applied Math., Vol. XIX, AMS, Providence, R.I., 1967, pp. 19–32.