Using the single operation of flowchart composition, classes of flowchart schemes are generated from arbitrary sets of graphs representing basic control structures. Starting from the appropriate initial structures then yields the conventional class of structured program graphs and provides a way of unifying earlier formulations for such graphs. It also allows for the generalization of the process to include any control structures which might subsequently be required.

The key results are achieved by relating each flowchart to its CGK-graph [1], which is devoid of process nodes but retains all the essential structure. Using such graphs removes the need to factor out irrelevant detail from like flowgraphs and simplifies proofs. Problems associated with equivalence (in the regular language sense) and the unfolding of flowgraphs are considered, as well as matters relating to the restructuring of flowgraphs and checking that the arbitrary graphs are well structured.

The paper is well written, the proofs are easy to follow, and the results provide a general framework in which to study all flowchart schemes. One mild criticism is the authors’ passing reference to a “companion paper” and a “subsequent paper,” no details of which are given. If these papers exist, but are not yet published formally, maybe mention could have been made of preliminary versions (i.e., technical reports).