Fixed but with old versions of pdflatex (fancy symbols not allowed in bookmarks).

--- a/src/HOL/Algebra/Module.thy	Sat May 03 13:31:07 2003 +0200
+++ b/src/HOL/Algebra/Module.thy	Mon May 05 10:53:27 2003 +0200
@@ -139,7 +139,9 @@
   finally show ?thesis .
 qed
 
-subsection {* Every Abelian Group is a $\mathbb{Z}$-module *}
+subsection {* Every Abelian Group is a Z-module *}
+
+(* Not all versions of pdflatex permit $\mathbb{Z}$ in bookmarks. *)
 
 text {* Not finished. *}