public static function getDefaultRendererForViewer(PhabricatorUser $viewer)
 {
     $is_unified = $viewer->compareUserSetting(PhabricatorUnifiedDiffsSetting::SETTINGKEY, PhabricatorUnifiedDiffsSetting::VALUE_ALWAYS_UNIFIED);
     if ($is_unified) {
         return '1up';
     }
     return null;
 }