diff -r 53e7969a92eb -r c8a41699e613 src/lemon/concept/graph_component.h
--- a/src/lemon/concept/graph_component.h	Fri Dec 03 12:19:26 2004 +0000
+++ b/src/lemon/concept/graph_component.h	Mon Dec 06 00:30:44 2004 +0000
@@ -14,7 +14,7 @@
  *
  */
 
-///\ingroup concept
+///\ingroup graph_concepts
 ///\file
 ///\brief The graph components.
 
@@ -28,22 +28,32 @@
 namespace lemon {
   namespace concept {
 
+    /// \addtogroup graph_concepts
+    /// @{
 
     /****************   Graph iterator concepts   ****************/
 
-    /// Skeleton class for graph Node and Edge
+    /// Skeleton class for graph Node and Edge types
 
-    /// \note Because Node and Edge are forbidden to inherit from the same
-    /// base class, this is a template. For Node you should instantiate it
-    /// with character 'n' and for Edge with 'e'.
+    /// This class describes the interface of Node and Edge (and UndirEdge
+    /// in undirected graphs) subtypes of graph types.
+    ///
+    /// \note This class is a template class so that we can use it to
+    /// create graph skeleton classes. The reason for this is than Node
+    /// and Edge types should \em not derive from the same base class.
+    /// For Node you should instantiate it with character 'n' and for Edge
+    /// with 'e'.
 
-    template<char _selector>
+#ifndef DOXYGEN
+    template <char _selector = '0'>
+#endif
     class GraphItem {
     public:
       /// Default constructor.
       
-      /// @warning The default constructor sets the item
-      /// to an undefined value.
+      /// \warning The default constructor is not required to set
+      /// the item to some well-defined value. So you should consider it
+      /// as uninitialized.
       GraphItem() {}
       /// Copy constructor.
       
@@ -71,9 +81,17 @@
       ///
       bool operator!=(GraphItem) const { return false; }
 
-      // Technical requirement. Do we really need this?
-      //      bool operator<(GraphItem) const { return false; }
+      /// Artificial ordering operator.
 
+      /// To allow the use of graph descriptors as key type in std::map or
+      /// similar associative container we require this.
+      ///
+      /// \note This operator only have to define some strict ordering of
+      /// the items; this order has nothing to do with the iteration
+      /// ordering of the items.
+      ///
+      /// \bug This is a technical requirement. Do we really need this?
+      bool operator<(GraphItem) const { return false; }
 
       template<typename _GraphItem>
       struct Constraints {
@@ -88,6 +106,7 @@
 	  //	  b = (ia == ib) && (ia != ib) && (ia < ib);
 	  b = (ia == ib) && (ia != ib);
 	  b = (ia == INVALID) && (ib != INVALID);
+	  b = (ia < ib);
 	}
 
 	const _GraphItem &ia;
@@ -95,6 +114,21 @@
       };
     };
 
+    /// A type describing the concept of graph node
+
+    /// This is an instantiation of \ref GraphItem which can be used as a
+    /// Node subtype in graph skeleton definitions
+    typedef GraphItem<'n'> GraphNode;
+
+    /// A type describing the concept of graph edge
+
+    /// This is an instantiation of \ref GraphItem which can be used as a
+    /// Edge subtype in graph skeleton definitions
+    typedef GraphItem<'e'> GraphEdge;
+
+
+    /**************** Basic features of graphs ****************/
+
     /// An empty base graph class.
   
     /// This class provides the minimal set of features needed for a graph
@@ -629,6 +663,11 @@
     };
 
 
+    /// Class describing the concept of graph maps
+
+    /// This class describes the common interface of the graph maps
+    /// (NodeMap, EdgeMap), that is \ref maps "maps" which can be used to
+    /// associate data to graph descriptors (nodes or edges).
     template <typename Graph, typename Item, typename _Value>
     class GraphMap : public ReadWriteMap<Item, _Value> {
     protected:      
@@ -804,6 +843,8 @@
       };
     };
 
+    /// @}
+
   }
 
 }