Edges of the first kind are called forward edges because their tail is listed beforetheir head in the vertex list 1→. . . i →j . . .→n defining the path; edges of thesecond kind are called backward edges because their tail is listed after their head inthe path list 1→. . . i ←j . . .→n. To illustrate, for the path 1→4→3←2→5→6of the last example, (1, 4), (4, 3), (2, 5), and (5, 6) are the forward edges, and (3, 2)is the backward edge
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