/**
  * @return ImsXmlReader
  */
 public function get_reader()
 {
     if (!empty($this->reader)) {
         return $this->reader;
     }
     if (!file_exists($this->path)) {
         return $this->reader = ImsXmlReader::get_empty_reader();
     }
     $path = $this->path;
     $ext = pathinfo($path, PATHINFO_EXTENSION);
     if ($ext == 'xml') {
         return $this->reader = new ImsQtiReader($path);
     } else {
         return $this->reader = ImsXmlReader::get_empty_reader();
     }
 }