anchor.diff:  
   %% @doc Send POST request and retrieve response.
   %%
  -%% @spec post(url(), [http_header()], binary(), Options) ->
  +%% @spec post(url(), [http_header()], iolist(), Options) ->
   %%   {ok, http_response()} | {error, Reason}
   
   post(URL, Headers, Body, Opts) ->
  
positive.code: send_http_body(Sock, <<>> = _Body) ->
    ok = send(Sock, "\r\n");
  send_http_body(Sock, Body) ->
    Length = integer_to_list(byte_size(Body)),
    ok = send(Sock, ["Content-Length: ", Length, "\r\n"]),
    ok = send(Sock, "\r\n"),
    ok = send(Sock, Body).
  
negative.code: %%
  %%   When `Body = none', no <i>Content-Length</i> is sent, of course.
  %%
  %% @spec send_http_body(socket(), iolist() | none) ->
  %%   ok
  
  send_http_body(Sock, none = _Body) ->
  