1. Wirth, N.: The development of programs by stepwise refinement. Comm. ACM. 14, 221?227 (1971).
2. Dijkstra, E. W.: Notes on structured programming. In Structured Programming. Academic Press (1972).
3. Hoare, C. A. R.: Notes on data structuring. Ibid.
4. Dahl, O.-J.: Hierachical program structures. Ibid.
5. Milner, R.: An algebraic definition of simulation between programs. CS 205 Stanford University, February 1971.