/// \ingroup graphs

   ///

   /// \brief A static directed graph class.

   ///

   /// \ref StaticDigraph is a highly efficient digraph implementation,

   /// but it is fully static.

   /// It stores only two \c int values for each node and only four \c int

   /// values for each arc. Moreover it provides faster item iteration than

   /// \ref ListDigraph and \ref SmartDigraph, especially using \c OutArcIt

   /// iterators, since its arcs are stored in an appropriate order.

   /// However it only provides build() and clear() functions and does not

   /// support any other modification of the digraph.

   ///

   /// This type fully conforms to the \ref concepts::Digraph "Digraph concept".

   /// Most of its member functions and nested classes are documented

   /// only in the concept class.

   ///

   /// \sa concepts::Digraph

     /// \brief Clear the digraph.

     ///

     /// This function erases all nodes and arcs from the digraph.

     void clear() {

       Parent::clear();

     }

     /// \brief Build the digraph copying another digraph.

     ///

     /// This function builds the digraph copying another digraph of any

     /// kind. It can be called more than once, but in such case, the whole

     /// structure and all maps will be cleared and rebuilt.

     ///

     /// This method also makes possible to copy a digraph to a StaticDigraph

     /// structure using \ref DigraphCopy.

     /// 

     /// \param digraph An existing digraph to be copied.

     /// \param nodeRef The node references will be copied into this map.

     /// Its key type must be \c Digraph::Node and its value type must be

     /// \c StaticDigraph::Node.

     /// It must conform to the \ref concepts::ReadWriteMap "ReadWriteMap"

     /// concept.

     /// \param arcRef The arc references will be copied into this map.

     /// Its key type must be \c Digraph::Arc and its value type must be

     /// \c StaticDigraph::Arc.

     /// It must conform to the \ref concepts::WriteMap "WriteMap" concept.

     ///

     /// \note If you do not need the arc references, then you could use

     /// \ref NullMap for the last parameter. However the node references

     /// are required by the function itself, thus they must be readable

     /// from the map.