anchor.diff:  
   format_path(Req, State = #state{backend = {_, _, _, BasePath}}) ->
     {Parts, Req2} = cowboy_req:path_info(Req),
  -  Path = list_to_binary(path_join(BasePath, Parts)),
  -  Req3 = cowboy_req:set_meta(backend_path, Path, Req2),
  -  next(Req3, State, fun append_qs/2).
  +  Path = path_join(Parts),
  +  Req3 = cowboy_req:set_meta(backend_path, path_join_base(BasePath, Path), Req2),
  +  Req4 = cowboy_req:set_meta(base_path, Path, Req3),
  +  next(Req4, State, fun append_qs/2).
   
   append_qs(Req, State) ->
     {Path, _} = cowboy_req:meta(backend_path, Req),
  
positive.code:   case cowboy_req:header(Header, Req) of
      {undefined, _} ->
        [Path] = cowboy_req:get([path], Req),
        {BackendPath, _} = cowboy_req:meta(backend_path, Req),
        [{Header, binary:part(Path, {0, backend_size(Path, BackendPath)})}];
      _ ->
        []
  
negative.code: 
  %% internal.
  
  path_join(undefined) ->
    <<>>;
  path_join(Parts) ->
    list_to_binary([<<"/">>, io_join(lists:reverse(Parts), <<"/">>, [])]).
  
  path_join_base(BasePath, <<>>) ->
    BasePath;
  path_join_base(<<>>, Path) ->
    Path;
  path_join_base(<<"/">>, Path) ->
    Path;
  path_join_base(BasePath, Path) ->
    <<BasePath/binary, Path/binary>>.
  
  io_join([], _Sep, Acc) ->
    Acc;
  