Because of the previously mentioned undirectedness of the graph, if (v, o) ∈ N u , then (u, -o) ∈ N v . However, it is often convenient to be able to enumerate the edges of the graph without double counting an edge and its reverse. For this reason, the representation comprises another list of size n whose u-th element is the first index i in N u such that N u [i] = (v, o) with either v > u or v = u and o > 0 3 . Such an edge (u, v, o) will be called a direct edge.