diff -r 1b7dc0728f5c -r aedb93833ea8 src/HOL/Library/Multiset.thy
--- a/src/HOL/Library/Multiset.thy	Tue Mar 04 16:38:21 2025 +0100
+++ b/src/HOL/Library/Multiset.thy	Tue Mar 04 16:58:46 2025 +0100
@@ -1337,7 +1337,7 @@
   qed
 qed
 
-lemma filter_mset_eq_mempty_iff: "filter_mset P A = {#} \<longleftrightarrow> (\<forall>x. x \<in># A \<longrightarrow> \<not>P x)"
+lemma filter_mset_eq_mempty_iff[simp]: "filter_mset P A = {#} \<longleftrightarrow> (\<forall>x. x \<in># A \<longrightarrow> \<not> P x)"
   by (auto simp: multiset_eq_iff count_eq_zero_iff)
 
 lemma filter_filter_mset: "filter_mset P (filter_mset Q M) = {#x \<in># M. Q x \<and> P x#}"