proper qualification needed due to shadowing on theory merge

--- a/src/HOL/Library/RBT.thy	Thu Nov 18 17:06:02 2010 +0100
+++ b/src/HOL/Library/RBT.thy	Thu Nov 18 18:53:36 2010 +0100
@@ -144,7 +144,7 @@
 
 lemma lookup_map [simp]:
   "lookup (map f t) k = Option.map (f k) (lookup t k)"
-  by (simp add: map_def lookup_RBT lookup_map lookup_impl_of)
+  by (simp add: map_def lookup_RBT RBT_Impl.lookup_map lookup_impl_of)
 
 lemma fold_fold:
   "fold f t = More_List.fold (prod_case f) (entries t)"