/*!
* @class DirectedGraph
*
* @extends Graph
*
* @brief A directed graph that stores the edges in an adjacency list.
*
* @signature template <[typename TCargo[, typename TSpec]]> class
* Graph<Directed<TCargo, TSpec> >;
*
* @tparam TCargo The cargo type that can be attached to the edges. Default:
* <tt>void</tt>.
* @tparam TSpec The specializing type. Default: <tt>Default</tt>. Use
* <tt>WithoutEdgeId</tt> here to omit edge ids. NB: if edges do
* not store ids then external property maps do not work.
*
* <img src="directedGraph.png" title="A directed graph." />
*/