--- a/ProgTutorial/Essential.thy	Wed Jun 02 07:05:29 2010 +0200
+++ b/ProgTutorial/Essential.thy	Wed Jun 02 07:19:58 2010 +0200
@@ -1291,7 +1291,7 @@
   \end{readmore}
 *}
 
-section {* Sorts (TBD) *}
+section {* Sorts (TBD)\label{sec:sorts} *}
 
 text {*
   Type classes are formal names in the type system which are linked to