  A line digraph $L(G) = (A, E)$ is the digraph constructed from the digraph $G
= (V, A)$ such that there is an arc $(a,b)$ in $L(G)$ if the terminal node of
$a$ in $G$ is the initial node of $b$. The maximum number of arcs in a line
digraph with $m$ nodes is $(m/2)^2 + (m/2)$ if $m$ is even, and $((m - 1)/2)^2
+ m - 1$ otherwise. For $m \geq 7$, there is only one line digraph with as many
arcs if $m$ is even, and if $m$ is odd, there are two line digraphs, each being
the transpose of the other.
