anchor.diff:  %% <ul>
   %%    <li>`{{digest, Realm}, true}': there is at least one valid user authenticated
   %%        with this `Realm'.</li>
  +%%    <li>`{{digest, Realm}, invalid}': there is at least one user offering
  +%%        an invalid authentication header for this `Realm'</li>
   %%    <li>`{{digest, Realm}, false}': there is at least one user offering 
   %%        an authentication header for this `Realm', but all of them have
   %%        failed the authentication.</li>
  
positive.code:                         Opaque -> ?call_notice("received invalid nonce", []);
                          _ -> ok
                      end,
                      not_found;
                  Method=='ACK' orelse Found=={Ip, Port} ->
                      CNonce = nksip_lib:get_value(cnonce, AuthHeader),
                      Nc = nksip_lib:get_value(nc, AuthHeader),
  
negative.code: %% @private
  -spec check_auth_header(nksip:optslist(), binary(), binary(), binary(), 
                              binary(), nksip:request()) -> 
      true | invalid | false.
  
  check_auth_header(AuthHeader, Resp, User, Realm, Pass, Req) ->
      #sipmsg{
  