diff -r 5b926cc36a4b -r ece1f8a3052d lemon/bits/graph_adaptor_extender.h
--- a/lemon/bits/graph_adaptor_extender.h	Tue Mar 16 21:12:10 2010 +0100
+++ b/lemon/bits/graph_adaptor_extender.h	Mon Apr 05 23:33:57 2010 +0200
@@ -181,6 +181,8 @@
     typedef _Graph Graph;
     typedef GraphAdaptorExtender Adaptor;
 
+    typedef True UndirectedTag;
+
     typedef typename Parent::Node Node;
     typedef typename Parent::Arc Arc;
     typedef typename Parent::Edge Edge;