1 PROCEDURE (G: reduced graph)
 2 Let R be an empty set
 3 Let P(u) precedessors of u
 4 //assign integer labels sinks and isolated vertices.
 5 FOR each u in G
 6     IF P(u) is empty
 7       Assign an integer i to u
 8       Increment i by 1
 9       Add v to U
10       Remove v from G
11     ENDIF
12 END LOOP
13
14 WHILE G not empty
15     //Find a set of unlabelled vertices
16     //that has no unlabelled predecessors
17     FOR each v in G
18       IF (v is not labelled && P(v) are labelled)
19         Add v to R
20         Remove v from G
21       ENDIF
22     END LOOP
23
24 ENDWHILE
25 END PROCEDURE