diff -r 3bb2ad8b1b37 -r 743f3603ba8b doc-src/IsarAdvanced/Codegen/Thy/examples/Codegen.hs
--- a/doc-src/IsarAdvanced/Codegen/Thy/examples/Codegen.hs	Tue Oct 16 17:06:21 2007 +0200
+++ b/doc-src/IsarAdvanced/Codegen/Thy/examples/Codegen.hs	Tue Oct 16 17:07:40 2007 +0200
@@ -10,8 +10,11 @@
 heada (x : xs) = x;
 heada [] = Codegen.nulla;
 
+null_option :: Maybe a;
+null_option = Nothing;
+
 instance Codegen.Null (Maybe a) where {
-  nulla = Nothing;
+  nulla = Codegen.null_option;
 };
 
 dummy :: Maybe Nat.Nat;