/**

    @defgroup gwrappers Wrapper Classes for Graphs

    \brief This group contains several wrapper classes for graphs

    @ingroup graphs

    The main parts of LEMON are the different graph structures, 

    generic graph algorithms, graph concepts which couple these, and 

    graph wrappers. While the previous notions are more or less clear, the 

    latter one needs further explanation. 

    Graph wrappers are graph classes which serve for considering graph 

    structures in different ways. 

    A short example makes this much 

    clearer. 

    Suppose that we have an instance \c g of a directed graph

    type say ListGraph and an algorithm 

    \code template<typename Graph> int algorithm(const Graph&); \endcode 

    is needed to run on the reversed oriented graph. 

    It may be expensive (in time or in memory usage) to copy 

    \c g with the reversed orientation. 

    In this case, a wrapper class is used, which 

    (according to LEMON graph concepts) works as a graph. 

    The wrapper uses 

    the original graph structure and graph operations when methods of the 

    reversed oriented graph are called. 

    This means that the wrapper have minor memory usage, 

    and do not perform sophisticated algorithmic actions. 

    The purpose of it is to give a tool for the cases when 

    a graph have to be used in a specific alteration. 

    If this alteration is obtained by a usual construction 

    like filtering the edge-set or considering a new orientation, then 

    a wrapper is worthwhile to use. 

    To come back to the reversed oriented graph, in this situation 

    \code template<typename Graph> class RevGraphWrapper; \endcode 

    template class can be used. 

    The code looks as follows 

    \code

    ListGraph g;

    RevGraphWrapper<ListGraph> rgw(g);

    int result=algorithm(rgw);

    \endcode

    After running the algorithm, the original graph \c g 

    is untouched. 

    This techniques gives rise to an elegant code, and 

    based on stable graph wrappers, complex algorithms can be 

    implemented easily. 

    In flow, circulation and bipartite matching problems, the residual 

    graph is of particular importance. Combining a wrapper implementing 

    this, shortest path algorithms and minimum mean cycle algorithms, 

    a range of weighted and cardinality optimization algorithms can be 

    obtained. 

    For other examples, 

    the interested user is referred to the detailed documentation of 

    particular wrappers. 

    The behavior of graph wrappers can be very different. Some of them keep 

    capabilities of the original graph while in other cases this would be 

    meaningless. This means that the concepts that they are models of depend 

    on the graph wrapper, and the wrapped graph(s). 

    If an edge of \c rgw is deleted, this is carried out by 

    deleting the corresponding edge of \c g, thus the wrapper modifies the 

    original graph. 

    But for a residual 

    graph, this operation has no sense. 

    Let us stand one more example here to simplify your work. 

    RevGraphWrapper has constructor 

    \code 

    RevGraphWrapper(Graph& _g);

    \endcode

    This means that in a situation, 

    when a <tt> const ListGraph& </tt> reference to a graph is given, 

    then it have to be instantiated with <tt>Graph=const ListGraph</tt>.

    \code

    int algorithm1(const ListGraph& g) {

    RevGraphWrapper<const ListGraph> rgw(g);

    return algorithm2(rgw);

    }

    \endcode

 */