anchor.diff:    case call(Req, State, request_id) of
       no_call ->
         next(Req, State, fun req_headers/2);
  +    ?HANDLE_ERROR(State);
       {{ReqName, ResName, ID}, Req2, HandlerState} ->
         State2 = State#state{handler_state = HandlerState,
                              req_headers = [{ReqName, ID}|ReqHeaders],
  
positive.code:   case call(Req, State, req_headers, ReqHeaders) of
      no_call ->
        next(Req, State, fun init_request/2);
      {ReqHeaders2, Req2, HandlerState} ->
        State2 = State#state{handler_state = HandlerState,
                             req_headers = ReqHeaders2},
  
negative.code:   res_headers = [] :: [{iolist(), iolist()}],
    res_has_body :: boolean()
  }).
  
  -define(HANDLE_ERROR(State),
    {error, ExitStatus, HandlerReq} when is_integer(ExitStatus) ->
      next(HandlerReq, State, ExitStatus)
  ).
  
  %% @doc Upgrade a HTTP request to the proxy protocol.
  %%
  