/*!

 \page maps How to write maps

 \section read-maps Readable Maps

 It is quite easy to write your own readmap for the edges or nodes of a graph.

 You can find some example below.

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

 \code

 struct MyMap 

 {

   typedef double ValueType;

   double operator[](Graph::EdgeIt e) const { return M_PI;}

 };

 \endcode

 Here is a more complex example. It provides a length function which is obtained

 from a base length function modified by a potential difference.

 \todo Please improve on the english. 

 \code

 class MyLengthMap 

 {

   const Graph::EdgeMap &ol;

   const Graph::NodeMap &pot;

 public:

   typedef double ValueType;

   double operator[](Graph::EdgeIt e) const {

     return ol.get(e)-pot.get(v)-pot.get(u);

   }

   MyComplexMap(const Graph::EdgeMap &o,const Graph::NodeMap &p) :

     ol(o), pot(p);

 };

 \endcode

 \todo Don't we need \e to \e require a 'typedef xxx KeyType' tag, as well?

 */