/*!

\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?

*/