/// \e source and \e target nodes. This digraph can be read with the

   /// \e source and \e target nodes. This digraph can be read with the

   /// following code:

   /// following code:

   ///

   ///

   ///\code

   ///\code

   ///ListDigraph digraph;

   ///ListDigraph digraph;

   ///ListDigraph::Node src, trg;

   ///ListDigraph::Node src, trg;

   ///digraphReader(digraph, std::cin).

   ///digraphReader(digraph, std::cin).

   ///  arcMap("capacity", cap).

   ///  arcMap("capacity", cap).

   ///  node("source", src).

   ///  node("source", src).

   ///  node("target", trg).

   ///  node("target", trg).