anchor.diff:  
   
   code_change( OldVsn, State, Extra ) ->
  -    call_impl( code_change, [ OldVsn, State#state.context, Extra ], State ).
  +    optional_call_impl(
  +      code_change, [ OldVsn, State#state.context, Extra ], State,
  +      { ok, State }
  +     ).
   
   
   %% ===================================================================
  
positive.code:     when is_binary( Driver ) or
           is_list( Driver )
      -> lurch_device_ext.
  
  call_impl_ctx( Func, Params, State ) ->
      call_impl_ctx( Func, Params, State, State#state.id ).
  
negative.code:         fun( good_event, _Ctx ) -> { ok, value, context1 };
             ( bad_event, _Ctx ) -> { error, reason, context2 }
          end ),
      meck:expect( ?IMPL, stop_driver, fun( _, _ ) -> ok end ),
  
      #state{ id = make_ref(), impl = ?IMPL, owner = self() }.
  