private function isUpdatingLatestVersion(Tracker_Artifact $artifact)
 {
     $valid_unmodified = true;
     $valid_match = true;
     if (isset($_SERVER['HTTP_IF_UNMODIFIED_SINCE'])) {
         $client_version = strtotime($_SERVER['HTTP_IF_UNMODIFIED_SINCE']);
         $last_version = $artifact->getLastUpdateDate();
         $valid_unmodified = $last_version == $client_version;
     }
     if (isset($_SERVER['HTTP_IF_MATCH'])) {
         $client_version = $_SERVER['HTTP_IF_MATCH'];
         $last_version = $artifact->getVersionIdentifier();
         $valid_match = $last_version == $client_version;
     }
     return $valid_unmodified && $valid_match;
 }