/// An empty graph class.

     /// The directed graph concept

     /// This class provides all the common features of a graph structure,

     /// This class describes the \ref concept "concept" of the

     /// however completely without implementations and real data structures

     /// immutable directed graphs.

     /// behind the interface.

     /// All graph algorithms should compile with this class, but it will not

     /// run properly, of course.

     /// It can be used for checking the interface compatibility,

     /// Note that actual graph implementation like @ref ListGraph or

     /// or it can serve as a skeleton of a new graph structure.

     /// @ref SmartGraph may have several additional functionality.

     /// 

     /// Also, you will find here the full documentation of a certain graph

     /// feature, the documentation of a real graph imlementation

     /// like @ref ListGraph or

     /// @ref SmartGraph will just refer to this structure.

     /// \todo A pages describing the concept of concept description would

     /// \sa concept

     /// be nice.