Edge _findEdge(Node u,Node v, Edge prev = INVALID) 

     {

       int e = (prev.n==-1)? nodes[u.n].first_out : edges[prev.n].next_out;

       while(e!=-1 && edges[e].target!=v.n) e = edges[e].next_out;

       prev.n=e;

       return prev;

     }

     /// Finds an edge between two nodes.

     /// Finds an edge from node \c u to node \c v.

     ///

     /// If \c prev is \ref INVALID (this is the default value), then

     /// it finds the first edge from \c u to \c v. Otherwise it looks for

     /// the next edge from \c u to \c v after \c prev.

     /// \return The found edge or \ref INVALID if there is no such an edge.

     ///

     /// Thus you can iterate through each edge from \c u to \c v as it follows.

     /// \code

     /// for(Edge e=G.findEdge(u,v);e!=INVALID;e=G.findEdge(u,v,e)) {

     ///   ...

     /// }

     /// \endcode

     /// \todo Possibly it should be a global function.

     Edge findEdge(Node u,Node v, Edge prev = INVALID) 

     {

       return _findEdge(u,v,prev);

     }