/**

 \page maps1 Maps I.

 In the previous section we discussed graph topology. That is the skeleton a complex

 graph represented data-set needs. But how to assign the data itself to that skeleton?<br>

 Here come the \b maps in.

 \section maps_intro Introduction to maps

 Maps play a central role in LEMON. As their name suggests, they map a certain range of <i>keys</i> to certain <i>values</i>.

 In LEMON there is many types of maps. Each map has two typedef's to determine the types of keys and values, like this:

 \code

   typedef Edge Key;

   typedef double Value;

 \endcode

 (Except matrix maps, they have two key types.)

 To make easy to use them - especially as template parameters - there are <i>map concepts</i> like by graph classes.

 <ul>

 <li>\ref ReadMap - values can be red out with the \c operator[].

 \code value_typed_variable = map_instance[key_value]; \endcode

 </li>

 <li>\ref WriteMap - values can be set with the \c set() member function.

 \code map_instance.set(key_value, value_typed_expression); \endcode

 </li>

 <li>\ref ReadWriteMap - it's just a shortcut to indicate that the map is both

 readable and writable. It is delivered from them.

 </li>

 <li>\ref ReferenceMap - a subclass of ReadWriteMap. It has two additional typedefs

 <i>Reference</i> and <i>ConstReference</i> and two overloads of \c operator[] to

 providing you constant or non-constant reference to the value belonging to a key,

 so you have a direct access to the memory address where it is stored.

 </li>

 <li>And there are the Matrix version of these maps, where the values are assigned to a pair of keys.

 The keys can be different types. (\ref ReadMatrixMap, \ref WriteMatrixMap, \ref ReadWriteMatrixMap, \ref ReferenceMatrixMap)

 </li>

 </ul>

 \section maps_graph Graphs' maps

 Every \ref MappableGraphComponent "mappable" graph class has two public templates: NodeMap<VALUE> and EdgeMap<VALUE>

 satisfying the \ref GraphMap concept.

 If you want to assign data to nodes, just declare a NodeMap with the corresponding

 type. As an example, think of a edge-weighted directed graph.

 \code ListGraph::EdgeMap<int>  weight(graph); \endcode

 You can see that the map needs the graph hows edges will mapped, but nothing more.

 If the graph class is extendable or erasable the map will automatically follow

 the changes you make. If a new node is added a default value is mapped to it.

 You can define the default value by passing a second argument to the map's constructor.

 \code ListGraph::EdgeMap<int>  weight(graph, 13); \endcode

 But keep in mind that \c VALUE has to have copy constructor.

 Of course \c VALUE can be a rather complex type.

 For practice let's see the following template function (from \ref maps_summary "maps-summary.cc" in the \ref demo directory)!

 \dontinclude maps_summary.cc

 \skip template

 \until }

 The task is simple. We need the summary of some kind of data assigned to a graph's nodes.

 (Whit a little trick the summary can be calculated only to a sub-graph without changing

 this code. See \ref SubGraph techniques - that's LEMON's true potential.)

 And the usage is simpler than the declaration suggests. The compiler deduces the

 template specialization, so the usage is like a simple function call.

 \skip std

 \until ;

 Most of the time you will probably use graph maps, but keep in mind, that in LEMON maps are more general and can be used widely.

 If you want some 'real-life' examples see the next page, where we discuss \ref algorithms

 (coming soon) and will use maps hardly.

 Or if you want to know more about maps read these \ref maps2 "advanced map techniques".

 */