src/HOL/Tools/Function/scnp_reconstruct.ML

 structure Multiset_Setup = Theory_Data
 (
   type T = multiset_setup option
   val empty = NONE
   val extend = I;
-  fun merge (v1, v2) = if is_some v1 then v1 else v2
+  val merge = merge_options
 )
 
 val multiset_setup = Multiset_Setup.put o SOME
 
 fun undef _ = error "undef"