diff -r cb34f5f49a08 -r ec32cdaab97b src/HOL/Probability/Stopping_Time.thy
--- a/src/HOL/Probability/Stopping_Time.thy	Tue Dec 19 14:51:27 2017 +0100
+++ b/src/HOL/Probability/Stopping_Time.thy	Tue Dec 19 13:58:12 2017 +0100
@@ -1,6 +1,6 @@
 (* Author: Johannes Hölzl <hoelzl@in.tum.de> *)
 
-section {* Stopping times *}
+section \<open>Stopping times\<close>
 
 theory Stopping_Time
   imports "HOL-Analysis.Analysis"