/// Reserve memory for nodes.

     /// Using this function, it is possible to avoid superfluous memory

     /// allocation: if you know that the graph you want to build will

     /// be large (e.g. it will contain millions of nodes and/or edges),

     /// then it is worth reserving space for this amount before starting

     /// to build the graph.

     /// \sa reserveEdge()

     void reserveNode(int n) { nodes.reserve(n); };

     /// Reserve memory for edges.

     /// Using this function, it is possible to avoid superfluous memory

     /// allocation: if you know that the graph you want to build will

     /// be large (e.g. it will contain millions of nodes and/or edges),

     /// then it is worth reserving space for this amount before starting

     /// to build the graph.

     /// \sa reserveNode()

     void reserveEdge(int m) { arcs.reserve(2 * m); };