// make an edge for a given Grpah
Edge mkEdge(Graph g, Vertex v, Vertex w)
{
   assert(validV(g,v) && validV(g,w));
   Edge e = {v,w}; // struct assignment
   // or  Edge e; e.v = v; e.w = w;
   return e;
}

// is a vertex valid in a given Graph?
static int validV(Graph g, Vertex v)
{
   return (g != NULL && v >= 0 && v < g->nV);
}