code: private function _generateIndex ($group, $id)
  	{
  		switch ($group) {
  			case 'custom':
  				$last = DateTimeHelper::toIso8601(
  					SitemapRecord::find()->one()->dateUpdated
  				);
  				$pages = 1;
  				break;
  
  			case 'sections':
  				$last = $this->_getUpdated(Entry::instance(), $id);
  				$pages = $this->_getPageCount(Entry::instance(), $id);
  				break;
  
  			case 'categories':
  				$last = $this->_getUpdated(Category::instance(), $id);
  				$pages = $this->_getPageCount(Category::instance(), $id);
  				break;
  
  			default:
  				$last = DateTimeHelper::currentUTCDateTime()->format('c');
  				$pages = 1;
  		}
  
  		for ($i = 0; $i < $pages; ++$i)
  		{
  			$sitemap = $this->_document->createElement('sitemap');
  			$this->_index->appendChild($sitemap);
  
  			$loc = $this->_document->createElement(
  				'loc',
  				$this->_indexUrl($group, $id, $i)
  			);
  			$sitemap->appendChild($loc);
  
  			$lastMod = $this->_document->createElement('lastmod', $last);
  			$sitemap->appendChild($lastMod);
  		}
  	}
docs: @param $group
  @param $id
  
  @throws \yii\base\Exception