src/Pure/Isar/code_unit.ML

   in
     thms
     |> burrow_thms (canonical_tvars thy purify_tvar)
     |> map (canonical_vars thy purify_var)
     |> map (canonical_absvars purify_var)
-    |> map Drule.zero_var_indexes
+    |> Drule.zero_var_indexes_list
   end;
 
 
 (* const aliasses *)