protected function _init($xml)
 {
     try {
         $this->setXml(new SimpleXMLElement($xml, 0, false));
     } catch (\Exception $e) {
         throw new \RuntimeException('An error occurred while trying to load XML string ' . $xml . ' : ' . $e->getMessage());
     }
     if (Helper::readProtocol($this->xml)) {
         $this->setProtocol(Helper::readProtocol($this->xml));
     }
     if (Helper::readVersion($this->xml)) {
         $this->setVersion(Helper::readVersion($this->xml));
     }
     if (Helper::readNamespaces($this->xml)) {
         $this->setNamespaces(Helper::readNamespaces($this->xml));
     }
     return $this;
 }