anchor.diff:      {ok, ResReq} =
           case xprof_tracer_handler:capture_stop(MFA) of
               ok ->
  -                cowboy_req:reply(204, Req);
  +                cowboy_req:reply(204, ?HDR_NO_CONTENT, Req);
               {error, not_found} ->
                   cowboy_req:reply(404, Req)
           end,
  
positive.code:     {ok, ResReq} =
          case xprof_tracer:monitor(Query) of
              ok ->
                  cowboy_req:reply(204, Req);
              {error, already_traced} ->
                  cowboy_req:reply(204, Req);
              _Error ->
                  cowboy_req:reply(400, Req)
          end,
  
negative.code: -export([init/3,handle/2,terminate/3]).
  
  -behavior(cowboy_http_handler).
  
  %% In case an XHR receives no content with no content-type Firefox will emit
  %% the following error: "XML Parsing Error: no root element found..."
  %% As a workaround always return a content-type of octet-stream with
  %% 204 No Content responses
  -define(HDR_NO_CONTENT, [{<<"Content-type">>, <<"application/octet-stream">>}]).
  
  %% Cowboy callbacks
  
  