diff -r d75cd481f8d9 -r 3ab140184a14 src/HOL/Data_Structures/Sorting.thy
--- a/src/HOL/Data_Structures/Sorting.thy	Sat Sep 22 16:03:31 2018 +0200
+++ b/src/HOL/Data_Structures/Sorting.thy	Sun Sep 23 12:50:12 2018 +0200
@@ -270,7 +270,7 @@
 subsubsection "Functional Correctness"
 
 lemma mset_merge_adj:
-  "\<Union># image_mset mset (mset (merge_adj xss)) = \<Union># image_mset mset (mset xss)"
+  "\<Union># (image_mset mset (mset (merge_adj xss))) = \<Union># (image_mset mset (mset xss))"
 by(induction xss rule: merge_adj.induct) (auto simp: mset_merge)
 
 lemma mset_merge_all: