diff -r cccbfa567117 -r b69e4da2604b src/HOL/Library/AList_Mapping.thy
--- a/src/HOL/Library/AList_Mapping.thy	Fri Jun 06 18:36:29 2025 +0100
+++ b/src/HOL/Library/AList_Mapping.thy	Mon Jun 09 22:14:38 2025 +0200
@@ -22,7 +22,7 @@
   by transfer simp
 
 lemma is_empty_Mapping [code]: "Mapping.is_empty (Mapping xs) \<longleftrightarrow> List.null xs"
-  by (cases xs) (simp_all add: is_empty_def null_def)
+  by (cases xs) (simp_all add: is_empty_def)
 
 lemma update_Mapping [code]: "Mapping.update k v (Mapping xs) = Mapping (AList.update k v xs)"
   by transfer (simp add: update_conv')