src/Pure/Proof/extraction.ML

--- a/src/Pure/Proof/extraction.ML	Thu Aug 26 16:56:45 2010 +0200
+++ b/src/Pure/Proof/extraction.ML	Thu Aug 26 17:01:12 2010 +0200
@@ -204,7 +204,7 @@
      realizers = Symtab.merge_list (eq_set (op =) o pairself #1) (realizers1, realizers2),
      defs = Library.merge Thm.eq_thm (defs1, defs2),
      expand = Library.merge (op =) (expand1, expand2),
-     prep = (case prep1 of NONE => prep2 | _ => prep1)};
+     prep = if is_some prep1 then prep1 else prep2};
 );
 
 fun read_condeq thy =