/**
  * Append childnode to document
  * @param ZXmlTag|DOMNode|mixed $a_tag
  * @return ZXmlDocument
  */
 public function &append($a_tag)
 {
     if ($a_tag instanceof ZXmlTag) {
         $this->m_document->appendChild($a_tag->node());
     } else {
         if ($a_tag instanceof DOMNode) {
             $this->m_document->appendChild($a_tag);
         }
     }
     return $this;
 }