template <class IncidenceGraph, class Buffer, class BFSVisitor,
            class ColorMap>
void breadth_first_visit
    (const IncidenceGraph& g,
     typename graph_traits<IncidenceGraph>::vertex_descriptor s,
     Buffer& Q, BFSVisitor vis, ColorMap color)
{

    function_requires< IncidenceGraphConcept<IncidenceGraph> >();
    typedef graph_traits<IncidenceGraph> GTraits;
    typedef typename GTraits::vertex_descriptor Vertex;
    typedef typename GTraits::edge_descriptor Edge;
    function_requires< BFSVisitorConcept<BFSVisitor, IncidenceGraph> >();
    function_requires< ReadWritePropertyMapConcept<ColorMap, Vertex> >();
    typedef typename property_traits<ColorMap>::value_type ColorValue;
    typedef color_traits<ColorValue> Color;
    typename GTraits::out_edge_iterator ei, ei_end;

    put(color, s, Color::gray());             vis.discover_vertex(s, g);
    Q.push(s);
    while (! Q.empty()) {
    Vertex u = Q.top(); Q.pop();            vis.examine_vertex(u, g);
         for (tie(ei, ei_end) = out_edges(u, g); ei != ei_end; ++ei) {
         Vertex v = target(*ei, g);            vis.examine_edge(*ei, g);
        ColorValue v_color = get(color, v);
        if (v_color == Color::white()) {      vis.tree_edge(*ei, g);
          put(color, v, Color::gray());       vis.discover_vertex(v, g);
          Q.push(v);
        } else {                              vis.non_tree_edge(*ei, g);
          if (v_color == Color::gray())       vis.gray_target(*ei, g);
          else                                vis.black_target(*ei, g);
        }
      } // end for
       put(color, u, Color::black());          vis.finish_vertex(u, g);
   //} // end while
    } // breadth_first_visit