added attribute "simp" to filter_mset_eq_mempty_iff

--- a/NEWS	Tue Mar 04 16:38:21 2025 +0100
+++ b/NEWS	Tue Mar 04 16:58:46 2025 +0100
@@ -20,7 +20,7 @@
   - Removed lemmas.
       size_multiset_sum_mset[simp]
   - Added lemmas.
-      filter_mset_eq_mempty_iff
+      filter_mset_eq_mempty_iff[simp]
       filter_mset_mono_strong
       filter_mset_sum_list
       set_mset_sum_list[simp]

--- 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#}"