diff -r 9e71155e3666 -r 0135a0c77b64 src/HOL/Probability/Stopping_Time.thy
--- a/src/HOL/Probability/Stopping_Time.thy	Fri Sep 24 13:40:14 2021 +0200
+++ b/src/HOL/Probability/Stopping_Time.thy	Fri Sep 24 22:23:26 2021 +0200
@@ -143,8 +143,9 @@
 lemma stopping_time_less_const:
   assumes T: "stopping_time F T" shows "Measurable.pred (F t) (\<lambda>\<omega>. T \<omega> < t)"
 proof -
-  guess D :: "'t set" by (rule countable_dense_setE)
-  note D = this
+  obtain D :: "'t set"
+    where D: "countable D" "\<And>X. open X \<Longrightarrow> X \<noteq> {} \<Longrightarrow> \<exists>d\<in>D. d \<in> X"
+  using countable_dense_setE by auto
   show ?thesis
   proof cases
     assume *: "\<forall>t'<t. \<exists>t''. t' < t'' \<and> t'' < t"