anchor.diff:    filter_headers(Headers, [Header|Acc]).
   
   %% TODO support more combos
  +normalize_backend(Conn) when is_pid(Conn) ->
  +  Conn;
   normalize_backend({Proto, Host, Port, Path}) ->
     {Proto, Host, Port, Path};
   normalize_backend({Proto, Host, Port}) when is_atom(Proto) andalso is_integer(Port) ->
  
positive.code:   {ok, Req2} = cowboy_req:reply(StatusCode, Req),
    terminate(Req2, State).
  
  terminate(Req, State = #state{env = Env, conn = Conn}) ->
    proxy_terminate(Req, State),
    ok = gun:close(Conn),
  
negative.code:   ],
    case gun:open(Host, Port, Opts) of
      {ok, Conn} ->
        next(Req, State#state{conn = Conn}, fun format_path/2);
      {error, Reason} ->
        error_terminate(Req, State, error, Reason, open_connection, 2, 502)
    end.
  
  format_path(Req, State = #state{backend = {_, _, _, BasePath}}) ->
  