“…Let T be a spanning tree of F 18 , as shown by dart lines in Figure 1. p is defined by φ = 0 on T and φ = (1, 0), (0, 1), (0, −1), (0, 1), (0, −1), (1, 1), (0, 0), (1, 0), (0, 0), and (1, 0) on the cotree arcs (1, 2), (2, 3), (3,4), (4, 5), (5,6), (1, 7), (7,14), (13,8), (14,9), and (11,18), respectively. By [23, Theorem 3.1], we have the following lemma.…”