Each vertex in LineGraph[g] corresponds to an edge in g.
For an undirected graph g, two vertices in LineGraph[g] are adjacent if their corresponding edges share a common vertex.
For a directed graph g, two vertices in LineGraph[g] are adjacent if their corresponding edges are connected, i.e. the target of one edge is the source of the other edge.
The vertices in LineGraph[g] are taken to be successive integers starting at 1.
