diff -r 98ddf8d5fda9 -r 71fd280363d5 lemon/bits/graph_adaptor_extender.h
--- a/lemon/bits/graph_adaptor_extender.h	Tue Dec 20 17:36:41 2011 +0100
+++ b/lemon/bits/graph_adaptor_extender.h	Tue Dec 20 17:38:19 2011 +0100
@@ -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;