fix document generation

--- a/src/HOL/SET-Protocol/ROOT.ML	Sat Feb 14 01:24:01 2009 -0800
+++ b/src/HOL/SET-Protocol/ROOT.ML	Sat Feb 14 06:53:28 2009 -0800
@@ -6,4 +6,5 @@
 Root file for the SET protocol proofs.
 *)
 
+no_document use_thy "Nat_Int_Bij";
 use_thys ["Cardholder_Registration", "Merchant_Registration", "Purchase"];