1. W. D. Maurer,A theory of computer instructions, Jour. ACM, Vol. 13, No 2, 226–235, (1966).
2. Z. Pawlak,About the notion of a program. A manuscript received from the author, January, 1967.
3. Wang Hao,A survey of mathematical logic, Sc. Press-Peking..., Amsterdam, (1964).
4. A. Schurmann,Application of graphs to [...] loops in a program, Infor. Control, 7 (1964), 275–282.
5. A. Grzegorczyk,Zarys logiki matematycznej [Outline of mathematical logic], PWN, Warsaw (1961).