diff -r f13796496e82 -r 5a4280946a25 src/HOL/Data_Structures/RBT_Set.thy
--- a/src/HOL/Data_Structures/RBT_Set.thy	Sun Apr 08 12:14:00 2018 +0200
+++ b/src/HOL/Data_Structures/RBT_Set.thy	Sun Apr 08 12:31:08 2018 +0200
@@ -262,7 +262,7 @@
 proof (standard, goal_cases)
   case 1 show ?case by simp
 next
-  case 2 thus ?case by(simp add: isin_set)
+  case 2 thus ?case by(simp add: isin_set_inorder)
 next
   case 3 thus ?case by(simp add: inorder_insert)
 next