/*!

 \page maps How to write your own maps

 \section read-maps Readable Maps

 The readable maps are very frequently used as the input of the

 algorithms.  For this purpose the most straightforward is to use the

 maps provided by Hugo's graph structres. Very often however, it is more

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

 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::Edge e) const { return M_PI;}

 };

 \endcode

 An alternative way to define maps. For this, \c MapBase seems to

 be a better name then \c NullMap

 \code

 struct MyMap : public MapBase<Edge,double>

 {

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

 };

 \endcode

 Or if we had \c KeyType and \c ValueType

 \code

 struct MyMap : public MapBase<Edge,double>

 {

   ValueType operator[](KeyType 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.

 \code

 class MyLengthMap 

 {

   const Graph::EdgeMap &ol;

   const Graph::NodeMap &pot;

 public:

   typedef double ValueType;

   double operator[](Graph::Edge 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 Please improve on the english. 

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

 */