src/HOL/Library/Multiset.thy

--- a/src/HOL/Library/Multiset.thy	Thu Aug 03 11:38:55 2017 +0200
+++ b/src/HOL/Library/Multiset.thy	Thu Aug 03 23:03:44 2017 +0200
@@ -3458,8 +3458,7 @@
 
 end
 
-lemma [code]: "sum_mset (mset xs) = sum_list xs"
-  by (induct xs) simp_all
+declare sum_mset_sum_list [code]
 
 lemma [code]: "prod_mset (mset xs) = fold times xs 1"
 proof -