/*!

\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?

*/