src/Pure/axclass.ML

 
 fun cert_classrel thy raw_rel =
   let
     val string_of_sort = Syntax.string_of_sort_global thy;
     val (c1, c2) = pairself (Sign.certify_class thy) raw_rel;
-    val _ = Sign.primitive_classrel (c1, c2) (Theory.copy thy);
+    val _ = Sign.primitive_classrel (c1, c2) thy;
     val _ =
       (case subtract (op =) (all_params_of thy [c1]) (all_params_of thy [c2]) of
         [] => ()
       | xs => raise TYPE ("Class " ^ string_of_sort [c1] ^ " lacks parameter(s) " ^
           commas_quote xs ^ " of " ^ string_of_sort [c2], [], []));