map_default: more explicit scope;

--- a/src/Pure/General/table.ML	Thu Aug 07 21:07:55 2008 +0200
+++ b/src/Pure/General/table.ML	Thu Aug 07 21:07:57 2008 +0200
@@ -250,7 +250,7 @@
 fun update_new (key, x) tab = modify key (fn NONE => x | SOME _ => raise DUP key) tab;
 fun default (key, x) tab = modify key (fn NONE => x | SOME _ => raise SAME) tab;
 fun map_entry key f = modify key (fn NONE => raise SAME | SOME x => f x);
-fun map_default (key, x) f = modify key (fn NONE => f x | SOME x => f x);
+fun map_default (key, x) f = modify key (fn NONE => f x | SOME y => f y);
 
 
 (* delete *)