/**
\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

*/