Index: lemon/bits/graph_extender.h
===================================================================
--- lemon/bits/graph_extender.h	(revision 57)
+++ lemon/bits/graph_extender.h	(revision 77)
@@ -21,4 +21,5 @@
 
 #include <lemon/bits/invalid.h>
+#include <lemon/bits/utility.h>
 
 #include <lemon/bits/map_extender.h>
@@ -334,4 +335,6 @@
     typedef GraphExtender Digraph;
 
+    typedef True UndirectedTag;
+
     typedef typename Parent::Node Node;
     typedef typename Parent::Arc Arc;