/**

 \page maps2 Maps II.

 Here we discuss some advanced map techniques. Like writing your own maps or how to

 extend/modify a maps functionality with adaptors.

 \section custom_maps Writing Custom ReadMap

 \subsection custom_read_maps Readable Maps

 Readable maps are very frequently used as the input of an

 algorithm.  For this purpose the most straightforward way is the use of the

 default maps provided by LEMON's graph structures.

 Very often however, it is more

 convenient and/or more efficient to write your own readable map.

 You can find some examples below. In these examples \c Graph is the

 type of the particular graph structure you use.

 This simple map assigns \f$\pi\f$ to each edge.

 \code

 struct MyMap 

 {

   typedef double Value;

   typedef Graph::Edge Key;

   double operator[](Key e) const { return M_PI;}

 };

 \endcode

 An alternative way to define maps is to use MapBase

 \code

 struct MyMap : public MapBase<Graph::Edge,double>

 {

   Value operator[](Key e) const { return M_PI;}

 };

 \endcode

 Here is a bit more complex example.

 It provides a length function obtained

 from a base length function shifted by a potential difference.

 \code

 class ReducedLengthMap  : public MapBase<Graph::Edge,double>

 {

   const Graph &g;

   const Graph::EdgeMap<double> &orig_len;

   const Graph::NodeMap<double> &pot;

 public:

   Value operator[](Key e) const {

     return orig_len[e]-(pot[g.target(e)]-pot[g.source(e)]);

   }

   ReducedLengthMap(const Graph &_g,

                    const Graph::EdgeMap &_o,

                    const Graph::NodeMap &_p)

     : g(_g), orig_len(_o), pot(_p) {};

 };

 \endcode

 Then, you can call e.g. Dijkstra algoritm on this map like this:

 \code

   ...

   ReducedLengthMap rm(g,len,pot);

   Dijkstra<Graph,ReducedLengthMap> dij(g,rm);

   dij.run(s);

   ...

 \endcode

 */