tuned proof

--- a/src/HOL/Relation.thy	Mon Mar 24 14:08:20 2025 +0100
+++ b/src/HOL/Relation.thy	Mon Mar 24 14:09:05 2025 +0100
@@ -217,7 +217,7 @@
   by (simp add: reflp_on_mono_strong le_fun_def)
 
 lemma reflp_on_subset: "reflp_on A R \<Longrightarrow> B \<subseteq> A \<Longrightarrow> reflp_on B R"
-  by (auto intro: reflp_onI dest: reflp_onD)
+  using reflp_on_mono_strong .
 
 lemma reflp_on_image: "reflp_on (f ` A) R \<longleftrightarrow> reflp_on A (\<lambda>a b. R (f a) (f b))"
   by (simp add: reflp_on_def)