Two new marking algorithms are described. They appear to be better than the ones of Martelli and Montanari [1,2]. To understand this paper it is necessary to know something about directed graphs. This very specific technical paper is not useful to the computer generalist.

AND/OR graphs can be seen as a generalization of directed graphs and therefore should be more powerful. This paper does not cover graphs with loops.

Overall, the paper is well done, but very narrow in scope. Some statements are not very clear. For example, the authors state, on p. 31, …“a non-terminal leaf node always has an infinite heuristic estimate…”It is not clear why this statement is made, nor its significance. The theorems and proofs seem consistent and correct.