anchor.diff:    case call(Req, State, forwarded_header_prefix) of
       no_call ->
         next(Req, State, fun request_id/2);
  +    ?HANDLE_ERROR(State);
       {{_, _, _, _} = Headers, Req2, HandlerState} ->
         ForwardedHeaders = format_forwarded_headers(Req, Headers),
         State2 = State#state{handler_state = HandlerState,
  
positive.code:   res_headers = [] :: [{iolist(), iolist()}],
    res_has_body :: boolean()
  }).
  
  %% @doc Upgrade a HTTP request to the proxy protocol.
  %%
  
negative.code:           State2 = State#state{backend = normalize_backend(Backend)},
            next(Req, State2, fun timeout/2)
        end;
      ?HANDLE_ERROR(State);
      {Backend, Req2, HandlerState} ->
        State2 = State#state{handler_state = HandlerState,
                             backend = normalize_backend(Backend)},
  