Fig. 4

Examples for self-complemental nodes and arcs. A self-complemental biarc [(ACG, CGT)] covered by a biEulerian circuit ([(GAC, ACG), (CGT, GTC)], [(ACG, CGT)], [(CGT, GTG), (CAC, ACG)]). The two directions of the bidirected circuit are drawn in blue ((GAC, ACG), (ACG, CGT), (CGT, GTG)) and green dotted ((CAC, ACG), (ACG, CGT), (CGT, GTC)). In a, the binode [ACG, CGT] is balanced, hence the circuit can enter it with some biarc, cover the self-complemental biarc, and then leave it via some other biarc. If, like in b, there was no other biarc to leave [ACG, CGT], then the graph would not be biEulerian, as the biarc [(GAC, ACG), (CGT, GTC)] cannot be used twice, even if the second use is in the other direction. Visually, the blue walk cannot use (CGT, GTC), since it was already used by the green dotted walk, and the green dotted walk cannot use (GAC, ACG), as it was already used by the blue walk