diff -r 5c8177fd1295 -r 8e6c973003c8 src/HOL/Tools/BNF/bnf_lfp_rec_sugar.ML
--- a/src/HOL/Tools/BNF/bnf_lfp_rec_sugar.ML	Wed Oct 20 17:11:46 2021 +0200
+++ b/src/HOL/Tools/BNF/bnf_lfp_rec_sugar.ML	Wed Oct 20 18:13:17 2021 +0200
@@ -143,7 +143,6 @@
 (
   type T = lfp_rec_extension option;
   val empty = NONE;
-  val extend = I;
   val merge = merge_options;
 );