diff -r dc376822c17d -r 6dd226d3dcba lemon/bits/edge_set_extender.h
--- a/lemon/bits/edge_set_extender.h	Mon Apr 05 23:41:05 2010 +0200
+++ b/lemon/bits/edge_set_extender.h	Tue Apr 06 07:04:38 2010 +0200
@@ -280,6 +280,8 @@
 
     typedef EdgeSetExtender Graph;
 
+    typedef True UndirectedTag;
+
     typedef typename Parent::Node Node;
     typedef typename Parent::Arc Arc;
     typedef typename Parent::Edge Edge;